Use CompositingStrategy.Offscreen in Modifier.punchHole (1/2)

This CL optimizes Modifier.punchHole by using an Offscreen composition
strategy (so that the layer can be blended against) rather than using
Canvas.withSaveLayer {} that reallocates an offscreen buffer every
frame.

Bug: 305195729
Test: PunchHoleTest
Flag: N/A
Change-Id: If26804247f36f763c812b007ecb114259c836e3b