Provide a way to customized the max enroll count for face

Bug: 412907224
Test: atest FaceFeatureProviderImplTest
Test: manual - no biometrics enrolled
             - click fingerprint item in device unlock page
	     - skip fingerprint enroll
	     - enroll face and the trigger back key in confirmation page
	     - skip fingerptint enroll again
	     - should return to device unlock page
Flag: EXEMPT bug fix
Change-Id: I36018b51b09378c388fe1872018d4dc522eb8657