Only initialize the drag session after we confirm we can handle the drag

- In ag/28686955 we moved the initialization of the drag session earlier
  in the flow to optimize the call to update the running task, but we
  should only need to initialize if there is valid drag data.

Flag: EXEMPT bugfix
Fixes: 383034393
Test: atest WMShellUnitTests
Change-Id: I8005cee8b7c0d4e58a173ea5de3b53e343f4523d