Add a new function in ResourcesProvider for SelfTargeting

This patch adds a new function to load a SelfTargeting overlay to be
a ResourcesProvider instance. If the app wants to overlay itself’s
resources, the first step is to call this new function to get a
ResourcesProvider instance and then add the ResourcesProvider in a
ResourcesLoader instance.
* ResourcesProvider#loadOverlay

Test: atest \
          OverlayHostTests \
          OverlayDeviceTests \
          SelfTargetingOverlayDeviceTests \
          OverlayRemountedTest \
          FrameworksServicesTests:com.android.server.om \
          CtsContentTestCases:android.content.om.cts \
          CtsContentTestCases:android.content.res.loader.cts \
          idmap2_tests

Bug: 205919743
Change-Id: Id54da44a54ee6cbc685ad486d1bf69ef388c33ae