Lower Suspense throttling heuristic to 300ms (#26803)

Summary:
Now that the throttling mechanism applies more often, we've decided to
lower this a tad to ensure it's not noticeable. The idea is it should be
just large enough to prevent jank when lots of different parts of the UI
load in rapid succession, but not large enough to make the UI feel
sluggish. There's no perfect number, it's just a heuristic.

DiffTrain build for commit f8de255e94.

Reviewed By: sammy-SC

Differential Revision: D45922304

fbshipit-source-id: 8a219f4fdf6a5a56ee4c3d7c5c130b68900b2937

packages/react-native/Libraries/Renderer/REVISION

@@ -1 +1 @@
-4bfcd02b2cebcb390f5aff0d7747c60a55012d5d
+f8de255e94540f9018d8196b3a34da500707c39b