Surface <overlayable> info in Java AssetManager

Add a new, hidden method to AssetManager to extract a mapping
overlayable name -> overlayable actor for all <overlayable> blocks in a
package. [This will eventually be used to check if the caller of the OMS
AIDL API is the registered actor for a given overlay.]

Also, teach AssetManager2 to not accept packages that re-use the same
overlayable name. [Such packages have always been ill-formed.]

Bug: 123894537
Test: make libandroidfw_tests
Change-Id: I1117fd3503f04fe4c73eb7114901e022508f4d9e