Further optimize Ime SubtypeSwitchingController

This further optimizes InputMethodSubtypeSwitchingController by removing
redundant string object allocations, instead relying on the CharSequence
compare method, and on keeping subtype hashcodes as ints instead of
strings (also improving set de-duplication).

Lastly, this removes most of the duplication from the test class.

Flag: EXEMPT cleanup
Bug: 347693610
Test: atest InputMethodSubtypeSwitchingControllerTest
Change-Id: I3a2dd0f0ecc429d08af0fb660ef56a94af04eb5d