Summary:
This diff updates the minsdkversion of RN OSS template to API level 21.
Changelog: [Android][Deprecated] Deprecate support of Android API levels 19 and 20. The new minSDK version will be 21+ moving forward.
Reviewed By: fkgozali
Differential Revision: D24379607
fbshipit-source-id: 6801cdcd363065807cdc11006bd94217f914fac7