RemoteControlClient metadata: add method for metadata of long type

Update to the RemoteControlClient.Editor API:
 - new method for setting metadata of type long
 - methods for setting metadata throw an exception if using
    the wrong key with the wrong method.

Note that this change will cause the Music2 application to
 throw an exception when it tries to set the duration as a string.

Change-Id: I17cff86467851a0aff313c07653e752ae6b6180f