Makes all custom caption settings unsearchable when custom captions are not active.

Unifies all custom caption controllers under a base controller, whose searchability is directly determined by CaptionHelper#getCustomCaptionAvailability().

Test: atest CaptionHelperTest
Bug: 353757664
Flag: com.android.settings.accessibility.fix_a11y_settings_search
Change-Id: If44e6eca4c72aa4413cd0a6e8735dbb22c7abe62