Add Maps entering PiP flicker test

Not all assertions will work initially, e.g. nav bar visibility changes
as the car mode overlay from Assistant is being rendered.

Also there are possible dialogs showing up when one opens Maps that
needs to be dismissed, asking for consulation in b/297893526.

Marking all tests that potentially fail as @Postsubmit too.

Bug: 282758103
Test: atest WMShellFlickerTestsPip:MapsEnterPipTest

Change-Id: I4d71264666eddc4318e06db6459b0caafc01e603