Adds MaskableIconDrawable class to android.graphics.drawable package

Test: manual test ag/1774462, cts tests ag/1813947

Bug: 32063838
Design doc: http://go/layered-icons-eng

- Reads in the mask path from config.
- If <maskable-icon> is dynamically created, clip the drawable using the mask path.
- Instead of using clipPath method, used BitmapShader for performance.

Change-Id: If5b4a24127bf823477c6c5d8a81d5093501d58e7