stagefright: Allow devices to use older widevine blobs

Devices may have widevine plugins built with an older MediaSource
interface.  Trying to use them fails because of the change in
interface in the object (it calls the destructor in the blob when
attempting to call setBuffers).

This commit restores the interface for ! QCOM_HARDWARE devices.

Fixes: JIRA-4944.

Change-Id: I4ce24a16f3eceab0c4def4b09850ed569eb392ab
(cherry picked from commit 82f7bb00)