Add transientTaskbarMargin above taskbar in overview

- Also removed transient_taskbar_two_panels_size and use sw720dp override instead, to be consistent with how we handle small vs large tablet in overview
- Also made isTransientTaskbar mockable so we can test that in DeviceProfileDumpTest

Bug: 260596114
Test: DeviceProfileDumpTest
Change-Id: I3c2236b95d91246e9581531478e3c97601cfb2ec