Fix DRM timer start for Widevine.

The DRM flag should always be true for WVMExtractor. The WVMExtractor always
turns on the DRM flag.  Before this change, it was reset to false in
MediaExtractor::Create.  After this change, MediaExtractor::Create will return
as soon as the WVMExtractor has been created.

Change-Id: I04949d2cd11b912bde61a106ee6b805932041c25
related-to-bug:6261699