IME API: rename IMS methods.

Incorporating API Review feedback:
* Use IMS.switchInputMethod(id) instead of IMS.setInputMethod()
* Use IMS.switchInputMethod(id, subtypeId) instead of
  IMS.setInputMethodAndSubtype()
* Renamed switchToLastInputMethod() to switchToPreviousInputMethod()

Bug: 73954137
Test: atest InputMethodServiceTest
Test: atest CtsInputMethodServiceHostTestCases
Change-Id: I6efd5ca473e33e6faeadb7eea7772b9d2b8ca12b