Fix uidmap in statsd.

Previously tried an optimization that results in corrupted proto
output. This changes to a safer approach of storing the snapshot data
in memory and only converting to proto output when the
ProtoOutputStream is provided.

Also fixes a security issue when trying to invoke triggerUidSnapshot
since we forgot to use SCS' permissions.

Test: Added a unit-test to verify output of StatsLogProcessor.
Bug: 76231867
Change-Id: Id410ce3505fda9d71caa71942ef3068b55872c66