Create source rect hint in PiP flicker test

Similar to PipUtils#getEnterPipWithOverlaySrcRectHint, we can create
valid source rect hint in PiP flicker test to ensure it's a valid one
despite the form factor the tests are running on, and it always matches
the given aspect ratio.

Flag: EXEMPT test
Bug: 326287104
Test: atest --iteration 2 \
      WMShellFlickerTestsPip1:AutoEnterPipWithSourceRectHintTest \
      WMShellFlickerTestsPip1:EnterPipOnUserLeaveHintTest
Change-Id: Icd417bd82339db2d6573e74147be1b984eb1f622