[Blinking Text Cursor Setting] Add ViewConfiguration API for getting text...

[Blinking Text Cursor Setting] Add ViewConfiguration API for getting text cursor blink interval from Settings

* Add a Secure setting for the text cursor blink rate.
* Define a new flagged getter API in ViewConfiguration that returns the setting value.
* Use this value in Editor to change the blink rate.
* Add testing methods for validating Editor behavior.
* Add a custom validator to limit acceptable values to a
  pre-defined range.
* Restore values to closest in set in SettingsBackupAgent.

Bug: 402134144
Bug: 385781377
Bug: 405251112
Flag: android.view.accessibility.text_cursor_blink_interval
Test: ViewConfigurationTest, EditTextTest, SettingsBackupAgentTest
Change-Id: I1dbee9c0461beccf649cb8b7f9f742c6c946d3f2