Skip dragging if app is already in default DesktopMode

This covers:
- PlatformScenarioTests "com.android.wm.shell.functional"
- WMShellFlickerTestsDesktopMode

Note that some tests are still failing, but this is the
first step to make sure it bypass the dragToDesktopMode
part (required in Tablet).

Bug: 378442632
Flag: EXEMPT test fix supporting default desktop mode
Test: atest PlatformScenarioTests --test-filter "com.android.wm.shell.functional"
Change-Id: I5330705db6c26b75c86eabcac368d5cbf254bebd