[AllAppsHeader] Add override system for implementer to add plugin rows to the header of all apps grid

Bug: b/352346748
Flag: com.google.android.apps.nexuslauncher.zero_state_tile
Test: Manual: http://recall/clips/395e9e4f-c20b-4a2b-bff1-a1ca9d5487d4
Change-Id: I65e2e9c68f05a6a387c05d5402ae3f5b2e2fcdba
2 files changed