[res] Simplify overlayable support in OverlayReferenceMapper

We don't have support for mulitple overlayable definitions per
package, and having that in the mock interface causes GC
pressure during boot for no reasons other than supporting test
inferfaces

Test: build + boot + \
  atest OverlayReferenceMapperTests AppsFilterImplTest
Flag: EXEMPT refactoring
Change-Id: I318d2118348ff484d9e9d4e27d50c9b1f464b7ca