ANDROID: uid_sys_stats: change to use rt_mutex

We see this happens multiple times in heavy workload in systrace
and AMS stuck in uid_lock.

Running process:        Process 953
Running thread: android.ui
State:  Uninterruptible Sleep
Start:
1,025.628 ms
Duration:
27,955.949 ms
On CPU:
Running instead:        system_server
Args:
{kernel callsite when blocked:: "uid_procstat_write+0xb8/0x144"}

Changing to rt_mutex can mitigate the priority inversion

Bug: 34991231
Bug: 34193533
Change-Id: I481baad840b7bc2dfa9b9a59b4dff93cafb90077
Test: on marlin
Signed-off-by: Wei Wang <wvw@google.com>