Improve tab drag experience and fix reentrancy issues

This CL has a lot of improvements but unfortunately can be hard to track
due to state transitions. The main changes are focused on reentrancy
problems with the move loop. Secondary changes are things like detecting
single window drag better or determining active tab state during
detaching/attaching.

CL does the following:
- Defer tab migration and target window transition until after the native window move loop has returned and unwound from the callstack. This prevents reentrancy issues during tab dragging operations.
- Releases window capture before invoking `RunWindowMoveLoop` so the
  OS can take exclusive mouse capture.
- Starts window drag immediately during session start when dragging an
  entire window (single tab in browser)
- Introduce `CompleteReattachment()` to perform tab migration and
  target updates after the move loop finishes instead of right after.
- Sets the active tab flag when inserting tabs into an empty target
  model or for the lead dragged tab.

Bug: b:535810736,b:536801500
Change-Id: I1919e65e393daf5a46a18f5e4b72ad0cf1073ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8208322
Reviewed-by: Fred Shih <ffred@chromium.org>
Commit-Queue: David Yeung <dayeung@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1675306}