[res] Get rid of String16 in AssetManager2

String16 is an obsolete class that was used in the old Android
code. New code prefers UTF8, and mixing it with String16 is slow
and inconvenient

+ get rid of one of the last usages of KeyedVector<>

Flag: EXEMPT minor refactoring
Test: atest libandroidfw_tests idmap2_tests aapt2_tests
Change-Id: I60b8f51488d0321718c23148caf196ae8e140897