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.

Bug: 72339335
Bug: 127641090
Test: ls /proc/uid/; compare with UIDs in /proc/uid_time_in_state
Change-Id: I0908f0c04438b11ceb673d860e58441bf503d478
Signed-off-by: Connor O'Brien <connoro@google.com>
[AmitP: Fix proc_fill_cache() now that upstream commit
        0168b9e3 ("procfs: switch instantiate_t to d_splice_alias()"),
        switched instantiate() callback to d_splice_alias()]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
[astrachan: Folded 97b7790f505e ("ANDROID: proc: fix undefined behavior
            in proc_uid_base_readdir") into this change]
Signed-off-by: Alistair Strachan <astrachan@google.com>