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