[path_provider] add getApplicationSupportDirectory

[collinjackson]

Fixes flutter/flutter#13474

This allows developers to store files that aren't user-generated in the correct folder on iOS.

Also updated the documentation:

1. push devs towards using getStorageDirectory on Android when storing user-generated content that should be visible to the user.
2. encourage the use of getTemporaryDirectory for storing caches of files that don't need to be backed up.

See https://developer.apple.com/icloud/documentation/data-storage/index.html

/cc @sroddy

[collinjackson]

It looks like the application support directory isn't automatically created.

I'm debating whether to create it automagically, but perhaps it is best to just return the path.

@mklim do you have any opinions?

[mklim]

I'm debating whether to create it automagically, but perhaps it is best to just return the path.

@mklim do you have any opinions?

I think it would be nice to create it automagically on the platform side. I wouldn't want to block for two async calls through the MethodChannel and back to check if it exists and then make it if it doesn't every time though.

[mklim]

nit:

Suggested change

	/// This directory is not guaranteed to exist and you can call `createSync` to
	/// This directory is not guaranteed to exist. You can call `createSync` to

[collinjackson]

Review feedback addressed, PTAL

[mklim]

LGTM

[nrkramer]

@collinjackson
As an update:

String databasePath = '${(await getApplicationSupportDirectory()).path}/likedMovies.db'; print(databasePath);

This works on Android and returns a path:
'/data/user/0/apps.blanket.showtap/files'

Yet still throws an exception:
E/MethodChannel#plugins.flutter.io/path_provider(18577): Failed to handle method call
E/MethodChannel#plugins.flutter.io/path_provider(18577): java.lang.IllegalStateException: Reply already submitted
E/MethodChannel#plugins.flutter.io/path_provider(18577): at io.flutter.embedding.engine.dart.DartMessenger$Reply.reply(DartMessenger.java:135)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler$1.notImplemented(MethodChannel.java:235)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at io.flutter.plugins.pathprovider.PathProviderPlugin.onMethodCall(PathProviderPlugin.java:44)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:222)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:96)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:643)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at android.os.MessageQueue.nativePollOnce(Native Method)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at android.os.MessageQueue.next(MessageQueue.java:326)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at android.os.Looper.loop(Looper.java:160)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at android.app.ActivityThread.main(ActivityThread.java:6669)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at java.lang.reflect.Method.invoke(Native Method)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
E/MethodChannel#plugins.flutter.io/path_provider(18577): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
E/DartMessenger(18577): Uncaught exception in binary message listener
E/DartMessenger(18577): java.lang.IllegalStateException: Reply already submitted
E/DartMessenger(18577): at io.flutter.embedding.engine.dart.DartMessenger$Reply.reply(DartMessenger.java:135)
E/DartMessenger(18577): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:240)
E/DartMessenger(18577): at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:96)
E/DartMessenger(18577): at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:643)
E/DartMessenger(18577): at android.os.MessageQueue.nativePollOnce(Native Method)
E/DartMessenger(18577): at android.os.MessageQueue.next(MessageQueue.java:326)
E/DartMessenger(18577): at android.os.Looper.loop(Looper.java:160)
E/DartMessenger(18577): at android.app.ActivityThread.main(ActivityThread.java:6669)
E/DartMessenger(18577): at java.lang.reflect.Method.invoke(Native Method)
E/DartMessenger(18577): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
E/DartMessenger(18577): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)