fix(android): rework longpress movement trigger

Fixes #6981.

Longpress menus were being triggered on any detected movement on keys on
Android. This would cause rapid typing to periodically fail as the user
would make a single-pixel movement while typing, which would then block
subsequent key events.

This issue was introduced in #6637, which resolved a somewhat related
problem with sticky longpress menus.

The fix is in three parts:

1. Remove the touch movement detection which caused the primary problem
2. Address some logic issues around visibility of `subkeysWindow`
3. Introduce a new 'scroll' gesture handler which detects a negative-y
   movement above a 5px threshold, to open the longpress menu.

The 5px threshold is the minimum default used by Keyman Engine for Web.
However, it may not be ideal, and we should consider moving to using the
0.25 x row height value that Keyman Engine for Web uses in a future
update. However, I do not consider this to be a reason to block this
fix, as it would require significant extra engineering.