Optimize InputMethodSubtypeSwitchingController

This avoids re-computing the list of items in the
InputMethodSubtypeSwitchingController every time the IME Switcher Menu
is requested, by instead storing the full list of items. These are still
updated whenever necessary (e.g. user, context or language change).

As this will be a superset of all the enabled IMEs and subtypes we could
use, this requires iterating and filtering when creating the list used
for switching, and the one used for the menu. However, this avoids
repeatedly calling into package manager to resolve the IME and subtype
label resources. With these lists being usually small, this should
overall improve performance.

To enable this, the logic for creating the list suitable for hardware
keyboard switching was merged into the main method. This presents a few
changes for the hardware keyboard list:
  * sorted based on the IME label, and IME id
  * removes duplicates based on the subtype hashcode

These two behaviours were already present on the main method, thus
combining them will provide more consistency in switching IMEs.

Flag: EXEMPT cleanup
Bug: 347693610
Test: atest InputMethodSubtypeSwitchingControllerTest
Change-Id: I11280dd485faa06676e8292b95cecdb312d92460