Add extras to ClipDescription

This is required to expand metadata capabilities of DragEvent.
Apps receiving ACTION_DRAG_* events have access to
ClipDescription, but not ClipData.

Adding extras to ClipDescription allows for a richer behavior of apps
responding to these events.

Bug: 25788641
Change-Id: I07e374f71d16f8441dc3a0b02c7d833e0139b74b