Add permission check to AccountManagerService to allow keyguard access.

When keyguard gets moved to its own process, it will need to access
specific user accounts in order to enable features such as the "forgot pattern"
button.  This adds an additional check to allow apps with the
INTERACT_ACROSS_USERS_FULL permission to work.

Change-Id: Id3107bc212ae3005870db781c32ae4405c068ad9