Support new merge strategies in BundleMerger.

Add a merge strategy to append two strings and to merge
two arrays by taking the union of their elements.

Bug: 395836346
Test: atest core/tests/coretests/src/android/os/BundleMergerTest.java
Flag: NONE trivial change
Change-Id: Idcf7362937d6774a232d1739e198ec5a62143a30