Rename and simplify DropPermissionHolder

Rename DropPermissionHolder to DropPermissions and move
all server-side implementation details into a separate class.
Rename DragEvent.getDropPermissionHolder to
requestDropPermissions and make it take the permissions
implicitly.

Change-Id: Ia83f7cb8af07ce13ba9536d24b0f9d63331d8736
12 files changed