Fix drag and drop URI permission grants

Currently cross app drag and drop with content URIs
only works for Owner. This is because the source Uid
is initialized after a Binder.clearCallingIdentity call.
The real source app identity is lost and things work
by accident for Owner only.

Bug: 25232077
Change-Id: Iaaaa735a04909a3b05fae7a7a421dbf02206cd68