Log stats for Optimize

OptimizeStats is crucial for us to tune our configuration for
optimization, which might affect the health of AppSearch and system
overall. Details can be found in Ib7ae475cc035d1b69969df1e22ac409895e0e3fa.

Also add sampling parameters in AppSearchConfig for:
1) initializeStats
2) searchStats
3) globalSearchStats
4) optimizeStats

Bug: 173532925
Bug: 175255572
Test: AppSearchStatsTest, FrameworksMockingServicesTests:AppSearchConfigTest, CtsAppSearchTestCases
Ignore-AOSP-First: AppSearch is not available on AOSP yet
Change-Id: Id61704407bdb40707cb8fa46b556024aea9f9083