Add corner radius at end assertion

Add an assertion to relevant PiP
flicker tests involving a PiP transition
that checks for a non-zero corner radius
at the final trace's layer level.

Currently adding the assertion to be in Postsubmit.
Also overriding the assertion for not appplciable tests,
such as ExitPipToAppTransition subclasses.

Bug: 240167840
Flag: EXEMPT bugfix
Test: atest WMShellFlickerTestsPip
Change-Id: Ifba1fc93aa465d579920b0f6e5f9432ee0dda254