ANDROID: proc: Add /proc/uid directory

Add support for reporting per-uid information through procfs, roughly
following the approach used for per-tid and per-tgid directories in
fs/proc/base.c.
This also entails some new tracking of which uids have been used, to
avoid losing information when the last task with a given uid exits.

Signed-off-by: Connor O'Brien <connoro@google.com>
Bug: 72339335
Test: ls /proc/uid/; compare with UIDs in /proc/uid_time_in_state
Change-Id: I0908f0c04438b11ceb673d860e58441bf503d478