Handle 9patches being used for frros

This adds an api that allows creation of nine patch frros.
We process them the same as aapt2 does so that
when they are used at runtime they work correctly.

API-Coverage-Bug: 314168567
Test: manual and automatic
Bug: 296324826
Change-Id: I40da020189e9ec914fbea0c17f181209347d83de