Frameworks/base: Use ArraySet more explicitly

In KeySetManagerService, use ArraySet more explicitly. Avoid for-each
loops.

Collections API methods on ArraySet are not very efficient. Iterators
incur two object allocations: a helper and the actual iterator object.
During boot, about 4.5K such calls are made. Using the ArraySet more
explicitly like an ArrayList/array avoids the overhead.

Bug: 19617481
Change-Id: I25df334fa1d4be3210667fb1404e3c43f2585049