Bug 1160563 - Part 2: Make ANDROID_ASSETS_DIRS a moz.build variable. r=gps

We have had singular ANDROID_ASSETS_DIR in Makefile.in for a while.
Fennec itself does not use the existing Makefile.in Android code, for
complicated historical reasons.

This makes the existing variable moz.build-only; generalizes the
existing variable to an ordered list; and adds the equivalent use of
the new list to the Fennec build, with a simple example asset.

This patch also updates the packager to include assets packed into the
gecko.ap_.  Without the packager change, the assets/ directory in the
ap_ gets left out of the final apk.  This whole approach is totally
non-standard but is more or less required to support our single-locale
repack scheme.

mobile/android/mach_commands.py

@@ -142,6 +142,7 @@ class MachCommands(MachCommandBase):
         srcdir('base/src/webrtc_video_capture/java', 'media/webrtc/trunk/webrtc/modules/video_capture/android/java/src')
         srcdir('base/src/webrtc_video_render/java', 'media/webrtc/trunk/webrtc/modules/video_render/android/java/src')
         srcdir('base/src/main/res', 'mobile/android/base/resources')
+        srcdir('base/src/main/assets', 'mobile/android/app/assets')
         srcdir('base/src/crashreporter/res', 'mobile/android/base/crashreporter/res')
 
         manifest_path = os.path.join(self.topobjdir, 'mobile', 'android', 'gradle.manifest')