[gatekeeperd] copy uid to local before passing to u_map

The compiler will issue the unaligned access instuctions
when reading from a packed struct. Since 'find' takes a
reference, if we pass the field directly it is removed
from its packed context and may be unreadable.

Read the field out directly from the packed struct and pass
in aligned to u_map to fix.

Bug: 22367550
Change-Id: Ia3b639c7518154ff5a2b7c233b752e154eab9aad