fix enrollment application permission check

Remove check associated with enrollment application UID, and allow
enrollment applications holding the KEYPHRASE_ENROLLMENT_APPLICATION
privledged permission to enroll.

Bug: 151405284
Test: Confirm enrollment from both enrollment application and active
voiceinteraction service. Confirm enrollment can still occurr after
system user switch.

Change-Id: I8686f705fe5405523004eef5de834282a7382464