Cleanup InputMethodServiceTest and SimpleIMS

This cleans up the InputMethodServiceTest code, as well as SimpleIMS and
its TestActivity, by clarifying method/field names, adding nullability
annotations, and introducing assertion messages.

Additionally removed the return value from showing/hiding the IME via
the InsetsController APIs, as these were always returning true.
Extracted separate methods for checking whether there is a navigation
bar, and setting the IME NavButtonFlags manually. Ensured the
countDownLatchForTesting is reset after usage. Explicilty handled
exceptions by failing the test with a clear message. Added the missing
testShowHideKeyboard_byInputMethodManager test. Removed an incorrect
assertion from testImeSwitchButtonLongClick.

Flag: EXEMPT testfix
Bug: 394328311
Test: atest InputMethodServiceTest
Change-Id: Id4aaa987114d4a90213f20a2e0895776b4eab0a5