a11y volume slider focus

With TalkBack on, the accessibility slider used to quickly drop
focus. Both on tap and on touch-and-explore.

Bug: b/285018188, b/201382499

Test: Manual. Turn on TalkBack. Press a physical volume button. Touch-and-explore to accessibility slider. The green rectangle signifying focus should stay on the slider.

Test: Manual. After step 2 above, touch-and-explore to background app,
should be able to interact with the background without Volume Dialog
taking over focus. Eventually (<= 16000ms) the dialog should dismiss.

Test: atest VolumeDialogImplTest

Change-Id: I2f5190a99a1b84ca4461b32ad1ba0d07460c9649