This is needed to prevent abuse of our service and to share icing
docids in framework, which are the resource we are most likely to
run out of.
Without this change, an app could use the platform backend to index
a very high number of documents and exhaust the docid limit, thereby
preventing any other app from using AppSearch.
Bug: 170371356
Test: New testcases added to AppSearchImplTest
Change-Id: I03ade35072bc69b84f8fcefed72b3c3bc2b8ee68
5e4a11cd10