Factor freqeuncy and mimetype of past sharings, foreground app into

sharesheet model. Promote most frequent sharable apps to tackle cold
start issue when device doesn't hold enough sharing history.

Bug: 149822311
Test: manual test Sharesheet ranking on device
Test: atest com.android.server.people.prediction.ShareTargetPredictorTest
Test: atest com.android.server.people.prediction.SharesheetModelHelperTest
Test: atest com.android.server.people.data.UsageStatsQueryHelperTest
Change-Id: I6eb764448c0619cbc3b320b5f6432f559d1a15e1