fixup! Add placeholder when media control title is blank

This is a squash of a revert of c332fc52
and the version of that "Add placeholder when media control title is
blank" commit from Google's android-platform-12.1.0_r20 tag.

The original commit contains a huge error. On the first condition
checking for if 'song' is null, the following line of code under it:
	song = metadata?.getString(MediaMetadata.METADATA_KEY_TITLE)
is changed to:
	song = metadata.getString(MediaMetadata.METADATA_KEY_TITLE)

That change causes crashes within SystemUI with the following null
pointer exception whenever an app tries to restore a previous media
playback session:
AndroidRuntime: FATAL EXCEPTION: SysUiBg
AndroidRuntime: Process: com.android.systemui, PID: 3016
AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.media.MediaMetadata.getString(java.lang.String)' on a null object reference
AndroidRuntime:        at com.android.systemui.media.MediaDataManager.loadMediaDataInBg(MediaDataManager.kt:584)
AndroidRuntime:        at com.android.systemui.media.MediaDataManager.access$loadMediaDataInBg(MediaDataManager.kt:96)
AndroidRuntime:        at com.android.systemui.media.MediaDataManager$loadMediaData$1.run(MediaDataManager.kt:354)
AndroidRuntime:        at android.os.Handler.handleCallback(Handler.java:938)
AndroidRuntime:        at android.os.Handler.dispatchMessage(Handler.java:99)
AndroidRuntime:        at android.os.Looper.loopOnce(Looper.java:201)
AndroidRuntime:        at android.os.Looper.loop(Looper.java:288)
AndroidRuntime:        at android.os.HandlerThread.run(HandlerThread.java:67

The commit from the android-platform-12.1.0_r20 tag correctly keeps the
former variant of the line, and, while the original commit has incorrect
changes to 'MediaDataManagerTest', it also has correct changes. So, this
commit reverts that line to that former variant and corrects changes to
MediaDataManagerTest.

Change-Id: I293a046814de67419fd97dbf6dc98dc3bcd5618c