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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1b3ccc22ec6f1c3df5dfe6d964853ca4d1d1f49a)
Merged-In: I8005cee8b7c0d4e58a173ea5de3b53e343f4523d
Change-Id: I8005cee8b7c0d4e58a173ea5de3b53e343f4523d