Fix several accessibility issues with BiometricPrompt

1) Ensure spaces can be tapped/double-tapped with talkback enabled
2) Do not explicitly set focus
3) Give empty spaces content description for talkback

Fixes: 132376836
Fixes: 132378618

Test: Enabled talkback, the above are fixed
Change-Id: Ic923ba8aed4922e852a8b82b6120644ca627d651