Fix UID tracking in ProcessCpuTracker.

Many processes are incorrectly listed under UID 0 (e.g. surfaceflinger)
because some "/proc/$PID/stat" files are with user "root".
The user of the folder "/proc/$PID/" is the correct one to use.

Bug: 119218763
Test: adb shell dumpsys batterystats -c | grep surfaceflinger
Change-Id: Ibfbe9b04c884bc9bde383629bcbba73f48ed34ee
(cherry picked from commit 7ead3ee4)