Adds APIs to let live caption can access the data in the DB (2/n)

1. Proposes the public APIs to get and the system APIs to set the
status of the captions and UI show/hide, and notify the status change.

2. Creates a new class, CaptioningManagerImpl, to get/set the value
of the secure setting keys through the A11yManagerService.

3. Adds a new permission, to protect the setter methods which can't
be used by 3rd party apps.

Bug: 136282740
Test: a11y CTS & unit tests
Change-Id: I591b98578dbb94ca8b5be8a553b81d23a143ee16
Ignore-AOSP-First: new permissions (not yet in AOSP)