logd: always report the UID of a log message

logd currently only reports the UID of a log message for 'privileged'
readers (those with a uid or gid of root, system, or log).  However,
UIDs are not particularly sensitive.  Much more importantly,
non-privileged readers can only see less messages from their own UID,
so this restriction is essentially a no-op, as those readers will
already know their own uid.

Test: liblog and logd unit tests
Change-Id: I9da7d15eb840ba3200128391e70d618eec79f988