Bind DropPermissions life cycle to Activity

Move requestDropPermissions from DragEvent to Activity.
Permissions will be granted using UriPermissionOwner
 associated with this activity and revoked when the activity
 is destroyed (if DropPermissions.release is not called before that).

Change-Id: Ic8f8fc3f56f57e83b9bc34ae8c96d82c2c9c4e1d