Fix race condition when using mPreserveTopNApps.

Bug: 191357172
Test: atest CacheOomRankerTest
Change-Id: I8cdf96ed1ddf56261e6ee953fb1078e961a0bc41
Merged-In: I8cdf96ed1ddf56261e6ee953fb1078e961a0bc41
(cherry picked from commit a0326c86ad318483ce05c30290fdba72c7b6e33d)
1 file changed