DRM framwork bug fix: add an API to release resources

When DrmManagerClient object is created and released many times,
the process suddenly crashes.
The case can happen when we make many thumbnails of
DRM-encrypted contents.

The problem is caused by shortage of file descriptors.
DrmManagerClient releases references of file descriptors
only when GC runs. So file descriptors are kept long time
even after the reference of DrmManagerClient are released.

By introducing DrmManagerClient#release() API,
the problem is solved. An application call this API
when we no longer need to use DrmManagerClient object.

Changes are made by SEMC and Sony.

Change-Id: Ie0bbc29cc33872449824285a8d67b1c3cdd8082b