Alternate bouncer focuses on FP indicator

Make the SFPS indicator focusable and update its label
to be more informative.

Fixes: 344763543
Test: atest SideFpsOverlayViewBinderTest
Test: manually enable talkback on UDFPS & SFPS devices.
Tap on notification intent to show alternate bouncer.
Observe information about fingerprint sensor is
announced.
Flag: EXEMPT bugfix

Change-Id: Iaaa8b00faa35917c9770631ddd54a1f1d87a47e6