Add fingerprint enroll reason

1) Existing metrics logging-ness can be derived from it
2) EnrollReason is clearer and can be used by other parts of the
   system, such as SysUI
3) Do not show progressbar in SysUI if it's "find sensor"

Bug: 179447737
Test: Builds
Change-Id: I84f82b5c81a668e11c1a380e6fa4e3080db53dc9