InputReader: Add API to get the last used input device

This will be used by the InputMethodManagerService to notify the IME of
the last input device type that was used to interact with Android when
the IME is shown. This API eliminates the need for IMMS to add another
global spy window to solve this problem.

Bug: 336615195
Test: atest inputflinger_tests
Change-Id: If594bd07bfd0a3cb542fc300854f1dd5717aeab2