Replace desktop mode screen size check with config property flag

The screen layout of a device can be modified by the user through
setting a higher or lower density, meaning the screen layout size can
change from the default value making it an unreliable gate. Calculating
the screen size in inches using the window metrics was also explored but
did not result in accureate values given current APIs and thus is also
an unreliable gate.

Instead introduce a new config property flag, to gate the feature,
which represents whether a device supports desktop mode. By default
this will be false but set to true in the config of supported devices.

Bug: 326957646
Fixes: 328684732
Test: atest WMShellUnitTests:DesktopModeWindowDecorViewModelTests
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Change-Id: I54329693746c6cb44ea4a54a6de00b33f9bc0a9c