Add captioning button to the Volume Panel

Plus a few changes to the infra:
 - fix volume panel styling;
 - rework ComponentsLayout to accomodate a separated buttons collection;
 - move FakeCaptioningRepository from SettingsLib to SystemUI to use it
   in the tests;

Flag: aconfig new_volume_panel DISABLED
Test: atest CaptioningViewModelTest
Test: atest DefaultComponentsLayoutManagerTest
Test: atest VolumePanelViewModelTest
Fixes: 324241246
Change-Id: I5277e92fe01b9e0ee65755e19f3c65ff7a710ed2