list caching tweaks

* stop background caching earlier - it's pointless to continue caching
  when the dataset changes and a new cursor is inbound. avoids creating
  unnecessary garbage.
* add explicit GCs to sidestep common cases that usually result in
  GC_FOR_ALLOC: after a query and after background caching

Bug: 9032162
Change-Id: Ic2e7819e6887b965515a63db3cac7e4dfdf70969
1 file changed