Add BiometricPrompt.Builder#setAllowBackgroundAuthentication TestApi

By default, auth via BiometricPrompt is not allowed unless the
caller is foreground. However, for cases like CTS, which may request
auth from the test itself (and not a test activity, which has no
way of getting access to protected TestApis), auth is requested
from background.

This would also allow us to easily add regression tests for the
security bug b/159249069

Bug: 163058911
Test: atest CtsBiometricsTestCases
Test: atest com.android.server.biometrics
Change-Id: I74bfdcd7989aa9256d1bf10eefae354983b42e6c