perf(animationFrames): uses fewer Subscription instances

[benlesh]

Leverages a single id variable instead of creating a new child Subscription for each scheduled animation frame. This is important because animation frames are scheduled very rapidly and this helps prevent CPU and GC thrashing.

Related #7018

[benlesh]

NOTE: Will do a follow up to remove animationFrameProvider.schedule, since it's not public and we're not using it after this PR.