Custom PlaybackControlView not update without exo_position and exo_duration

[baitouwei]

Issue description

Custom PlaybackControlView whitout position text and duration text then progress will not update.

Reproduction steps

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <com.google.android.exoplayer2.ui.DefaultTimeBar
        android:id="@id/exo_progress"
        android:layout_width="match_parent"
        android:layout_height="20dp">
    </com.google.android.exoplayer2.ui.DefaultTimeBar>
</FrameLayout>

1.Use above xml to custom PlaybackControlView
2.PlaybackControlView progress will not update

Version of ExoPlayer being used

r2.4.1

Device(s) and version(s) of Android being used

Geneymotion Nexus 5 6.0.0 API 23

[ojw28]

Duplicate of #2871. This will be fixed in the next release.