fp-image-device: Move fpi code into its own unit that can be compiled a part

In order to be able to test the private device code (used by drivers) we
need to have that split a part in a different .c file so that we can compile
it alone and link with it both the shared library and the test executables.

Redefine fp_image_device_get_instance_private for private usage, not to move
the private struct as part of FpDevice.

libfprint/meson.build

@@ -9,6 +9,7 @@ libfprint_sources = [
 libfprint_private_sources = [
     'fpi-assembling.c',
     'fpi-device.c',
+    'fpi-image-device.c',
     'fpi-ssm.c',
     'fpi-usb-transfer.c',
     'fpi-byte-reader.c',