ash: Implement keyboard shortcut to resize the PiP Window

Use "Search + X" to resize the PiP Window.
This shortcut behaves the same as double-tapping the PiP window.

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

- UXW review doc: https://docs.google.com/document/d/1E1tU2Tnv7urIcKeGjqjaHUTMz383l4YVSBfk5vq4L8U/edit?tab=t.0#heading=h.p6ytmfqg0fql
- Discussions we had we added a shortcut for PiP resizing: https://docs.google.com/document/d/1ZHLOd55rDCKDUSUeNbKEqOkwvskMQH1HepfreLL64KQ/edit?usp=sharing

    Push "Search + X" to resize the pip window properly.
    Show a description on the Key Shortcuts setting.

Bug: b:333656468
Test: ash_unittests --gtest_filter="*PipToggleResizeFeatureTest*"
Change-Id: I15f7919376abf327b9eab243c511ed277085906d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5874953
Commit-Queue: Kokoa Matsuda <komatsud@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Nergi Rahardi <nergi@google.com>
Cr-Commit-Position: refs/heads/main@{#1362270}