Actually depend on the headers from renderscript

Instead of having all the object compilations depend on a stamp file,
instead depend on the headers, which are already available in the build
graph.

This allows my RBE builds to just find the headers, instead of having to
walk the graph to find the other outputs.

Test: treehugger
Test: build a system image with RBE
Change-Id: I0a310229be26d7bd399e074a4350d5be038f8661