[Android Extension] Fix Activity leaking caused by DeviceCapabilities extension

The static XWalkMediaCodec singleton instance indirectly holds a
reference to Activity via XWalkExtensionManager, it would prevent the
Activity object from GCed and then leak it and all its resources.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2000
Merge pull request #2128 from hmin/activity-leak

[Android Extension] Fix Activity leaking caused by DeviceCapabilities ex...