Plumb UDFPS overlay "show reason" from FingerprintService to SysUI

Enables SysUI to draw enroll/auth-specific affordances

If the UI diverges, we can consider enroll or auth specific
UdfpsView subclasses(?) but currently enrollment just shows
an extra circle.

Fixes: 177275693
Test: atest com.android.systemui.biometrics

Change-Id: I5b2aaf00c55279a81348ec7a5dc387b6e62751c0