[automerger skipped] Revert^2 "exoplay min sdk bump from 19 to 21" am: 681dc31b97 -s ours

am skip reason: Merged-In I6a8b8451cc79517f2ad952d57106c38384091c6e with SHA-1 5687d2cb21 is already in history

Original change: https://android-review.googlesource.com/c/platform/external/exoplayer/+/3146303

Change-Id: I197c5c12385e688c3dcfd2aac9e9287b3adf2377
Signed-off-by: Automerger Merge Worker <[email protected]>