Fix AMediaDrm_getKeyRequest which fails clearkey InitDataParser::parse test.

AMediaDrm_getKeyRequest() passes mimeType to drm plugin's getKeyRequest()
based on the UUID. If the drm scheme is clearkey UUID, clearkey plugin will
return ERROR_DRM_CANNOT_HANDLE because it is expecting initDataType
(e.g. "cenc") instead of mimeType (e.g. "video/mp4").

Replace initDataType parameter in clearkey DrmPlugin::getKeyRequest() with
mimeType.

bug: 30790899
Change-Id: I60c77c458d8d836d3eb2f9f52296a86efa73a4eb