Fix Bluetooth Javadoc to clarify the maximum device name length.

The maximum length of a Bluetooth device name is 248 bytes using
UTF-8 encoding. Updated the Javadoc to clarify that the length is
limited by the number of UTF-8 bytes and not the number of characters.

Change-Id: I135671f5ee6c5eb6372f3fbbc5fccb02de65e6c4