Supply separate flags for onBuildTileIndex

Supply separate flags for onBuildTileIndex

This is a follow up to:
https://codereview.chromium.org/1401283003/

Supply separate flags for onBuildTileIndex

Since png and jpeg's implementations of onBuildTileIndex rely on
modifications to their underlying libraries, rather than whether we are
running on Android, use separate flags that can be disabled
independently.

This will allow us to easily turn off the feature. It also is a step
towards building and running on other platforms for testing (e.g.
valgrind/ASAN to find memory leaks etc).

BUG=skia:

Review URL: https://codereview.chromium.org/1402783008