Revert "exoplay min sdk bump from 19 to 21"

Revert submission 3143936-minsdkbump_aosp

Reason for revert: Droidmonitor created revert due to build breakages in b/349379920.

Reverted changes: /q/submissionid:3143936-minsdkbump_aosp

Change-Id: I74a7181887f1e555396d615d56a02bf9b85cb745
1 file changed