Revert "Revert "Update MbmsDownloadServiceBase to reflect implements IInterface""

This reverts commit 2c76358add49ff6064241c232c4700a5f46d5ed3.

Reason for revert: The api checker has now been fixed (https://android-review.googlesource.com/c/platform/cts/+/913473) so reapply the fix

Change-Id: Iadc1c756b98ab98c71e2e80f4c1d89331c9f20f2
2 files changed