Move a11y content change event throttle reduction behind a flag

We found that without throttle some apps become non-responsive.
For now, let's add a new flag and put the behavior behind it.

Bug: 297499858
Bug: 277305460
Test: With flag disabled, Facebook dispatches events every 100 ms.
Change-Id: I4542b33046edc6f2adb5167dc40021cf5108ef0f