Use button instead of seekbar when touch exploration is enabled

When touch exploration (such as talkback) is enabled, it's much
more difficult to accidentally exit the rear display mode, since
it requires two taps in order to activate a button.

Flag: EXEMPT bugfix
Fixes: 399023818
Test: atest RearDisplayInnerDialogDelegateTest
Test: atest RearDisplayCoreSTartableTest
Change-Id: If10e1116080ac3d83d27c7685c3b2915a5e68f14