fix(firefox-android): Support abstract sockets for RDB

[A1kmm]

New versions of firefox-android appear to use abstract unix sockets instead of filesystem sockets
for the debugging connection. This change ensures support for both the original filesystem
namespace and also the abstract namespace.

[coveralls]

Coverage remained the same at 100.0% when pulling ecfc06f on A1kmm:abstract-sockets into 9d28fb8 on mozilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling ecfc06f on A1kmm:abstract-sockets into 9d28fb8 on mozilla:master.

[rpl]

@A1kmm Thanks a lot for digging into this issue and preparing a fix for it (as well as including the needed changes to the unit tests, it has been very much appreciated!)

The changes included in the PR looks great to me, I'm going to give this branch a try locally on an android emulator asap (mainly to double-check the version of Firefox for Android that has introduced this change is Firefox for Android 63, at least based on the issues related to this that I found in a quick search on bugzilla, and also because this part is not covered by any functional test).

Nevertheless, in the meantime I'm marking this PR as reviewed right now.

[rpl]

Just verified this PR locally, and it fixes (as expected) the issue with "forwarding and connecting the Fennec remote debugging server socket" for both Firefox for Android 63.0.2 (the currently released version) and Firefox for Android Beta 64.0b7.

@A1kmm Thanks again for this PR!