Show freeform button only when dev option is on

Now some Android tablets have desktop mode, which shouldn't be
entered by clicking the freeform button in Overview. Therefore still
keep it hidden when the device claims the support of freeform window
management, but the user didn't enable the freeform dev options
explicitly.

This button should also be hidden when desktop mode is enabled.

Bug: 299362754
Test: The button is hidden when the dev option is disabled.
Change-Id: Id919af4fd1a027da0bb335c4c4b7303d15d689f8