Avoid codec/render race in AdaptivePlaybackTest

Code was sending frames off to render before inserting the timestamp
into a "to be rendered" timestamp list.  Recently, we've seen the renderer
running right away, flagging an error because the timestamp isn't (yet) in the list.

Bug: 188054645
Test: atest android.media.cts.AdaptivePlaybackTest
Change-Id: I63151267c274206396ed3e9e72138622e47f2432