Fixup for e4bc8990 (#8955)
HttpSource.fetchImage() uses Call.asObservableSuccess(), which
cancels the call on unsubscribe. This causes the call to be cancelled
before it is used, leading to a "java.net.SocketException: Socket is
closed" when trying to use the response in putImageToCache().
To fix this, use Call.awaitSuccess() via a new HttpSource.getImage()
suspending function. This addition to source-api is only intended for
app use, so it will not be added to the extensions-api stubs.
b4b3a4d286