optimization of gc load, reduce gc in some scenarios

1.NativeAlloc GC,Increase the count of the number of small native
memory allocations.

2.CollectTransition GC,App's allocations (since last GC) more than the threshold then do GC
when the app was in background. If not then don't do GC.

Expert in charge of ART, please evaluate this CL
and hope to merge it in master branch.

Bug:200116730
Change-Id: Id2977f05eb249691326955e6f2424d4e5e08b417
2 files changed