MediaMetadata2: Supplement Javadoc of each key

The Javadoc of the METADATA_KEY_* does not contain type information.
This CL adds '@see' in each key so that the developer can easily notice
what method should be used.

Bug: 73877547
Test: Builds successfully
Change-Id: I04f1b0badc1b6e7af6cb04923ef4f59328248b74