FingerprintService: check current user correctly

FingerprintService maintains a mCurrentUserId field which keeps track of
the active group/user id in the underlying daemon. This field, however,
can become invalid after the daemon crashes and before
FingerprintService reconnects to a new daemon. (mCurrentUserId is only
set upon updateActiveGroup().)

Previously, mCurrentUserId is wrongly used in the logic of
isCurrentUserOrProfile(), resulting in false negatives when
mCurrentUserId is invalid. In this CL, we use
ActivityManager.getCurrentUser() instead, which has no dependency on the
state of the daemon.

This will also reduce the complexity around mCurrentUserId, and
hopefully will make future clean-ups easier.

Test: manually kill daemon and verify fingerprint functions normally
Bug: 36152864
Change-Id: I4250fad0e7c88bde95a1da1c89e71b4931ee3b8a