The uid with the newest timestamp in mPendingStartActivityUids gets the best score.

getProcessStatesAndOomScoresForPIDs() returns the procstate and oom_adj
score of a list of uid. If OomAdjuster does not update procstate fast
enough after WindowManager switches activity to foreground, we use
mPendingStartActivityUids to save the uid that moves to foreground. Then
OomAdjuster removes the uid from mPendingStartActivityUids later.

Because OomAdjust has latencty, the uid may be delayed to be deleted
from mPendingStartActivityUids. This ends up with multiple uids in the
mPendingStartActivityUids. When these multiple uids compete for camera,
getProcessStatesAndOomScoresForPIDs() can not distinguish procstate and
scores for these uids.

This CL will compare the elapsedRealtime these uids are added
into mPendingStartActivityUids, the uid with the newest timestamp gets
the best score.

Bug: 218947003
Test: b/218947003 reproduce steps, fast swipe between Messaging and Camera
app for many times.

Change-Id: Ic28a617d234a20a3f8e33982c42e2e2c3ffd5dbd