Allow DEFAULT_INPUT_METHOD to capture audio during RTT call

Add content observer for Settings.Secure.DEFAULT_INPUT_METHOD,
which contains package name of current InputMethodService.

Bug: 147037345
Test: use voice input with Gboard 9.1.4 during a simulated RTT call.
Change-Id: I11e17b13513627c88ccb8e4db66c5359e9ee7f7b
3 files changed