Refactor DisplayList path caching.

This removes dependence on SkPath ptrs that HWUI does not control
the lifecycle of. This clears up some errors where the paths are
not generated from Java, but rather the Skia test suites.

Cherry-pick of a change that originally landed in master-skia and is
dependent on a skia merge (ag/655422).

Change-Id: I41b9797a2b0af5d6b4ea51891565469d4f1d832d