First pass at a new clipboard API.

ClipboardManager was in android.text(!!) so it needed to be moved
up to android.content to have access to the richer data types we
now need.

ClippedData is the data representation.  Still needs a lot of
fleshing out to allow holding more than one data type at a time
and perhaps conversions between them.  (MIME-oriented interrogation
and conversion will be done through ContentProvider, which needs
to grow an ability to report multiple MIME types and accept a
desired MIME type when a stream is being opened.)

Change-Id: Ifa51bedcd084a677813b255d171804e8496b0cb5