[automerger skipped] Revert "exoplay min sdk bump from 19 to 21" am: 496036f1dd -s ours

am skip reason: Merged-In I6a8b8451cc79517f2ad952d57106c38384091c6e with SHA-1 5687d2cb21 is already in history. Merged-In was found from reverted change.

Reverted change: https://android-review.googlesource.com/c/platform/external/exoplayer/+/3142607

Original change: https://android-review.googlesource.com/c/platform/external/exoplayer/+/3146841

Change-Id: Ie56cd390b2a8e9592c602a3a122aa83bd752f807
Signed-off-by: Automerger Merge Worker <[email protected]>