Remove interception capabilities in STL drag handler

This CL removes the interception capabilities of SceneTransitionLayout:
we won't be able to simply put a finger down during a transition to stop
it, and we instead now have to drag the touch slop to start a new
transition. Moreover, nested scrollables now have priority even during a
transition and will consume scroll events first before sending
unconsumed events to the nested scroll connection/STL draggable.

This capability was a nice-to-have that we added when STL was introduced
from the start. It has a high cost in terms of cognitive overload when
thinking about the participation of STL in the nested scroll chain, and
the gains (if any) are not obvious. Moreover, any complexity added in
the event chaining/interceptions can introduce subtle gesture bugs that
are hard to debug. For instance, b/379847834 seems to be mostly fixed
with this change, *probably* because of the removed pre-scroll
interception. In the worst case, we can still revert back this CL should
we realize that we really need these interception capabilities.

Because we don't intercept transitions, we also don't need to support
the "accelerated swipes" anymore.

Bug: 379281707
Bug: 379847834
Test: atest PlatformSceneTransitionLayouTests
Test: Manual, played with flexiglass and Communal
Flag: com.android.systemui.scene_container
Change-Id: I3128bfd6ec0f960b8b57ffb1b55908b4cc2c2ce7