Fix vorbis-specific workaround to only compare audio timestamps

This workaround was introduced in 2014 [1]. At the time it was correctly
only comparing audio timestamps because mAnchorTimeMediaUs only held
audio timestamps then. In 2017 [2], mAnchorTimeMediaUs started to hold
video timestamps too - at which point this comparison became invalid and
a bug was introduced that can lead to control-flow exiting too early
when handling an audio buffer with the same timestamp as the preceding
video buffer. If this happens at the start of playback, then the
**second** audio buffer timestamp is passed to
MediaClock::setStartingMediaTime, which can lead to incorrect values
being returned from NuPlayerRenderer::getCurrentPosition (when paused
at the start of playback).

[1] https://cs.android.com/android/_/android/platform/frameworks/av/+/eecb7805bbbb712925d4372c505f8c7f5c4fb5ed
[2] https://cs.android.com/android/_/android/platform/frameworks/av/+/17648f365e5914fa772bbfebaf9f5c6a37ddfb99

Test: atest MediaPlayerTest
Bug: 283882552
Bug: 282472894
Bug: 263063118
Change-Id: I7fbe6bdaff27c93f15d9d85902306a3f6e00b978