Keep screen on during fingerprint enrollment

If the user has a short display timeout set, it is possible for the
screen to turn off in the middle of enrollment, requiring the user
to turn the display back on.

Adding FLAG_KEEP_SCREEN_ON in onCreate fixes this behavior.

Change-Id: I6bfb071fa3004e409d094683e7c3ab3b0222cd57
REF: SAMBAR-213