Update MediaDrm documentation for provideKeyResponse

The documentation for MediaDrm#getKeyResponse says null is returned from
provideKeyResponse for requests of type streaming or release. This corrects
the documentation to indicate that an empty byte array is returned.

bug: 36093612
Change-Id: I2e4fc658d68d827b8a42fb7fa3ef30e35ee51dcf
1 file changed