SF Tracing: Provide a valid starting bounds

When generating layer bounds, we start with scaled display bounds
to set a crop on boundless layers (layers that scale to the display).
This breaks for layer trace generator since there are no displays
when recreating the trace. To fix this, we provide a valid display
bounds when starting the layer bounds calculations.

Test: sh $ANDROID_BUILD_TOP/frameworks/native/services/surfaceflinger/Tracing/tools/run.sh
Bug: 200284593
Change-Id: I54dde9f5e2f73b3b135b8997d6bfdbe394116e3a