Bug 854530 - pre: Remove unused MOZ_APP_ICON from mobile/android/base/Makefile.in.

MOZ_APP_ICON was a Makefile dependency not mentioned elsewhere in the
tree.
Bug 854530 - pre: Remove unused LINUX_BRANDING_FILES from mobile/android/branding/**/Makefile.in.

The text `LINUX_BRANDING` does not appear in the tree.  There are no
references to `fennec_48x48.png` or `fennec_72x72.png` outside of
mobile/android/base/Makefile.in, and that file explicitly copies the
referenced files (so does not depend on the deleted export rules).
Bug 854530 - Move resource declarations in mobile/android/base/Makefile.in to moz.build.

This patch:

* adds a new sandbox symbol, 'ANDROID_PACKAGE', to collect resource
  declarations;
* adds a new TreeMetadata subclass, AndroidPackageData to hold said
  declarations;
* adds a new backend AndroidPackageMakefileFragment class to write
  backend.mk rules to process the resources;
* moves lots of things out of mobile/android/base/Makefile.in and into
  moz.build;
* moves branding files out of android-resources.mn and into
  android-resources.build files.