Fix T-junctions in layers' generated meshes

bug:4128442

Currently we fix T-junctions for hardware layers, and for
Canvas.saveLayer if the transform isn't rectAsRect. Ideally, hardware
layers should lazily re-generate T-junction free geometry if the
transform becomes non-rectAsRect

Depends on frameworks/native change:
https://googleplex-android-review.googlesource.com/#/c/277668/
which adds Region::createTJunctionFreeRegion()

Change-Id: Ice75b4a1a4459ff835059ea81d70f7d1563fd96d