Fix scrubbing time views visibility

- Always make relevant button views GONE when scrubbing
- Show scrubbing time views if there is a prev/next slot (not only when
  there is a visible button there)

Fixes: 380056189
Test: visual, with and without scene container flag
Test: MediaControlPanelTest, MediaControlViewModelTest
Flag: EXEMPT bugfix
Change-Id: Ib1ef16bdb9df93d62773d23ab7a2904b008d0eca