float: Minor polishes for tablet mode.

Adds initial location for resizer, otherwise it would be 0,0 which is
rarely the case. Adds a minimum drag distance, prevents tiny swipes and
also single click/tap on the header from magneting to a different
corner. Fixes a constant that was moved wrongly. Sets the dragged
windows for the indicators, otherwise the wrong UI message will show.

Test: manual
Bug: 1336836c
Change-Id: Ia6fe580faf80746964ebb3458a6e2aa39b6385f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3857235


Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039409}