Framework: allow ResolverActivity to be specialized from other package

PackageManager already has hooks to allow the Disambiguation Dialog aka
ResolverActivity to be substituted at runtime by a custom activity
named via system property. Since the internal ResolverActivity already
has good extension points (for use by ChooserActivity), it is useful
to allow a customized ResolverActivity, provided in a separate package
to be able to extend the internal activity, thus preserving core
behavior as much as possible, and making forward porting easier.

This patch attempts to make as few changes to the core ResolverActivity
as possible to make up stream patching easy, and isolates most of the
needed code into a shim (ResolverProxy) which is mostly concerned with
changing the access modifier of key methods from package-private to
protected. This is necessary to allow overriding of those methods from
a subclass originating from a different class loader.

Change-Id: I17070964fca73f0d11b4fe8d809b2a775ba9e7e6