Fix overlay reference app visibility

Change get -> valueAt and get the actual base.apk ApkAssets for the
package. Also fixes overlay reference visibilty, which was broken with
the AppsFilter cache.

Bug: 184834206

Test: atest OverlayReferenceMapperTests
Test: atest OverlayActorVisibilityTest
Test: atest AppsFilterTest

Change-Id: I01e6a39b36a0e03c2cb9d48e705fe02c1a63c169