Up limit from 1fps to 3fps for takeScreenshot

Allow AccessibilityService instances to request screenshots more
frequently (up to three screenshots per second). This used to be capped
at one screenshot per second in Android R.

Test: cts
Bug: 188077373

Change-Id: I880e2d922db3ce2bd075efcb02466ea1f4914792
(cherry picked from commit 113436f4)