Enhance Javadoc of playback speed related APIs

This CL also makes throw IllegalArgumentException when controller
tries to set the speed to zero.

Bug: 131873534
Test: atest CtsMediaTestCases:android.media.cts.MediaSessionTest;
      atest CtsMediaTestCases:android.media.cts.MediaControllerTest;
      atest CtsMediaTestCases:android.media.cts.MediaBrowserTest;
      atest CtsMediaTestCases:android.media.cts.MediaSessionManagerTest;
Change-Id: I78f775df369de48b1ebda66fdb6e251b6e0865c1