Cancel every running stack animation safely

[OskarEichler]

Problem

StackAnimator.cancelAllAnimations() only clears its tracking maps; the animators keep running against views the caller may immediately destroy. cancelPushAnimations() iterates a live map while synchronous cancel/end listeners remove entries, so cancelling multiple pushes can also trigger concurrent modification or skip work.

Fix

• snapshot every running animator before mutating its registry
• clear all tracking maps before cancel-all so command-completion callbacks remain suppressed
• cancel the snapshots so every animator actually stops without iterating a listener-mutated map

Breaking changes

None. Cancellation now performs the action its API promises; normal completed animations and callback semantics are unchanged.

Test plan

• Added a cancel-all regression proving the animator receives cancel(), transition tracking clears, and the command callback stays suppressed.
• Added two-simultaneous-push coverage proving both animators are cancelled safely.
• Exact baseline fails because cancel-all never calls cancel(); fixed focused suite passes 14/14.
• Full Android unit suite passes: 699 successes, 0 failures, 2 skipped.
• git diff --check passes.