Improvements to Bugreporting API.

- Show a toast to indicate screenshot is taken.

BUG:149525300
Test: Flash and press bug report shortcut and check the toast
Change-Id: Ib210c8855d2b588b51f883e32070c8b748ea4e3f
Merged-In: Ib210c8855d2b588b51f883e32070c8b748ea4e3f