Add seek bar to QS Media Player

Bug: 150724977
Test: manual - play music and look for seek bar in QS media player
Test: maunal - play podcast and check that track position can be changed
Test: manual - play IHeartRadio and check that seek bar is gone
Test: adding SeekBarObserverTest and SeekBarViewModelTest
Change-Id: I98f32b939f2310e9eb492165f1fddfd7dee65a90