Bug 948465 - Part 1: Register resource://android/ pointing to Fennec APK root. r=mfinkle

Consumers can access Android assets (and resources) by using
resource://android/assets/FILENAME.

The resource substitution is registered at app-startup time so that the
substitution is in place before any add-on has started up: as documented
at [1], "[an] add-on will not receive xpcom-startup or app-startup
notifications".

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Guide/Receiving_startup_notifications
Bug 948465 - Part 2: Tests. r=mfinkle