Add toast messages when toggling mic from keyboard

At the moment, the "microphone mute" key (KEYCODE_MUTE) will just
toggle the microphone sensor without any feedback to the user;
this change will add a toast message that will inform the user of
the state the microphone is in.

Test: tested on device
Bug: 258853916
Change-Id: I98e04a11395ba0fcb88081b80ffd0098e2e2498c