Add InputMethodListListener interface.

It's for watching changes of installed IMEs.
The services in system_server can watch changes of installed IMEs by
implementing the new interface and register it to
InputMethodManagerInternal which can be obtained as LocalService.

Bug: 111905132
Test: Manually tested. The IME list on ChromeOS settings is updated
      correctly when installing/uninstalling IME apps.
Change-Id: I02a42d01c71f661ea877de472c5c9bd365014ac5
Merged-In: I02a42d01c71f661ea877de472c5c9bd365014ac5