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Add getters for the keys in each map stored in
PersistableBundle. This will allow users of
PersistableBundle to iterate over entries of
a specific type in PersistableBundle.

Also, add "using" statements to PersistableBundle
for std containers to reduce verbosity.

BUG: 27171235
Change-Id: Idd4b3bf713071ca8d8d023f1f83bdaf3ccbdd46f
TEST: libbinder compiles successfully.