Create Public API in AccessibilityManager to expose Audio description

enabled status.

1) Move ENABLE_ACCESSIBILITY_AUDIO_DESCRIPTION_BY_DEFAULT from
Settings.Global to Settings.Secure.
2) Add isAudioDescriptionEnabled public API to read the
Settings.Secure.ENABLED_ACCESSIBILITY_AUDIO_DESCRIPTION_BY_DEFAULT.

Test: local test
Test: atest AccessibilityManagerTest
Bug: 160823970
CTS-Coverage-Bug: 197311729
Change-Id: Iecf006bc8a1315414b37b6d644fc607b3d2aa03b