Fix start position issues and related flyout flickering.

- Cancel animations when explicitly moving the stack to the start position, so that the new translation doesn't get animated away.
- Set the flyout to 0 alpha before posting the animation, so it's invisible from the start.
- Tell the stack if the drag finished with a dismiss intent, so we can reset the drag flags.

Bug: 130441552
Test: atest SystemUITests
Change-Id: I31d753a6fb76468c4e2b68563f9c5f06fbd782a1