Improve performance of edge extension animation

- Run both edge extension creations in parallel to cut down wait time roughly in half for the default activity transition.
- Use surface scaling instead of a shader to "clamp" the edge extension.

Bug: 227449117
Test: atest CtsWindowManagerDeviceTestCases:ActivityTransitionTests
Change-Id: Iaab3978e6b6d8cb1db843a1c5ef0d28ea83bd2aa