Add ability to enforce limits on docs per package and size of doc.

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