ash: Organize the PipDoubleTapHandler class

This commit is for the future implementation.
The PipDoubleTapHandler class is removed,
and these feature is moved into the PipSizeSwitchHandler,
the inner class of the PipController class.

- Design doc: https://docs.google.com/document/d/1wTcLc9LEvDHQgi6K1FOiOxAYoUAf-yvwfLWCWiEq6pU/edit?usp=sharing&resourcekey=0-BZSEliNNgSFAnhKEJ69B5Q

	ash_unittests --gtest_filter="*PipDoubleTapHandler*"
	Double-tap the PiP window and resize properly

Bug: b:333656468
Test: ash_unittests --gtest_filter="*PipController*"
Change-Id: I4936f45c9b2144452c96245034be12ad7bb07917
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5859538
Commit-Queue: Kokoa Matsuda <komatsud@google.com>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Nergi Rahardi <nergi@google.com>
Reviewed-by: Kazuki Takise <takise@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1359726}