[PiP] Add feature flag to force default pip transition

This change introduces a new feature flag, `AllowEnhancedPipTransition`,
to disable the enhanced Picture-in-Picture transition animation on
desktop Android. When this flag is disabled, the transition will not use
the sourceRectHint parameter.

Enabled-by-default-reason: killswitch

Set No-Try due to broken win_optional_gpu_tests_rel that's unrelated to
this change.

No-Try: true

Bug: 357008916
Bug: 438743787
Change-Id: I2bab2423eb5d9a2797f76f77e681be57621f0753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6871268
Commit-Queue: Phil Yan <philyan@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1505531}