Handle API review comments from the council.

 * MtpEvent: Remove public constructor since properties cannot be set
   directly.
 * MtpEvent: Move event constants from MtpConstants to MtpEvent class.
 * getPartialObject64: the byte[] must have indices of Integer.MAX_VALUE
   at most. Document as such.
 * sendObjectInfo: what happens if the transfer doesn’t succeed? How is
   the app notified? If there is a reason for the failure communicated,
   how does the app find this out? Add docs.
 * Add isOperationSupported(int) and isEventSupported(int) helpers.

Change-Id: Ifd80016d2ddd3b66d5c45f6da76b6133f0c9a617
Fixes: 28146379