Enforce that app & launchable intent drags are mututally exclusive

- We should be checking that the given data is actually an app drag
  (ie. dragging an icon from taskbar) before setting app data, and
  as per comments in the field declarations, that we only set either
  appData or launchableIntent as non-null (with app-drags taking
  priority as if both are set (this behavior doesn't change in this CL)

Fixes: 411574401
Flag: EXEMPT bugfix
Test: atest WMShellUnitTests
Change-Id: I1c56c74db466021e125c500dd606eba21820ffb9