Add FingerprintEnroll API

Fixes: 65459785

Test: adb shell am start -a android.settings.FINGERPRINT_ENROLL
      Flow is correct with and without PIN/Pattern/Pass set up
Test: make -j56 SettingsUnitTests
      adb install -r out/target/product/walleye/data/app/SettingsUnitTests/SettingsUnitTests.apk
      adb shell am instrument -w -e class com.android.settings.fingerprint.FingerprintEnrollIntroductionTest 'com.android.settings.tests.unit/android.support.test.runner.AndroidJUnitRunner'

Change-Id: If987257943190eb91d5822e683657aea621d2206