Add ResourceLoader API with .apk and .arsc support

ResourceLoaders allow inserting another .apk/.arsc into AssetManager's
resource resolution search. The effect is similar to overlays,
where a entry of >= config later in the path list will return that
ApkAsset's resource value instead.

Because loading from an .arsc is supported, which doesn't contain
any actual files, ResourceLoader exposes loadDrawable and
loadXmlResourceParser to allow an application load those files from
anywhere or create them in code.

The data being loaded is either pushed into an .apk or .arsc that
mocks itself as the package being "overlaid" and is passed in
through ResourcesProvider, an interface with static methods that
supports loading from a readable path on disk or a FileDescriptor.

The APIs are accessed through a Context's getResources(), which
has been changed to be unique per "Context-scope", which is usually
the lifetime of the Java object. The exception is that Activities
who get their Resources object persisted across recreations
maintain that logic for persisting ResourceLoaders.

Bug: 135270223

Test: atest FrameworksResourceLoaderTests

Change-Id: I6929f0828629ad39a21fa155e7fec73bd75eec7d