In new direct share ranking, expand DS row as long as there are enough

targets parking in memory, even if targets are still loading on UI.

Backfill targets of top 8 apps into DS row instead of top 4, if the row
is not full.

Bug: 156470651
Test: atest CtsSharesheetTestCases:android.sharesheet.cts.CtsSharesheetDeviceTest
Test: atest com.android.internal.app.ChooserActivityTest
Change-Id: Idefd6c225fca723d6d266b78ef41a4201483c634