Fix heap trimming logic.

The new heap trimming logic introduced in the c/84301 was not very
good since the heap trim would get avoided since the daemon thread
was coming to the heap trimming code before the scheduled time.

The new logic is to do the heap trim if the last heap trim occurred
more than kHeapTrimWait ns ago.

Change-Id: I9d0e6766bf0c68e5f7fb15fb059140e1f1264216