fpi-device: Only use emulation mode by pre-loading a library

Rather than just do it in production if set, make it more difficult to
potentially inject the test mode.

So rely on a weak definition of fpi_device_emulation_mode_enabled() that
can be only set externally

libfprint/libfprint.ver

@@ -1,6 +1,7 @@
 LIBFPRINT_2.0.0 {
 global:
 	fp_*;
+	fpi_device_emulation_mode_enabled;
 local:
 	*;
 };