Use AIDL callbacks directly.

Bug: 210498497
Test: build cuttlefish with ag/16322062, watch logs
Change-Id: I1e0cd95f5f9a911564d14be2d8bbb328572de8c8
diff --git a/radio/aidl/compat/libradiocompat/modem/RadioModem.cpp b/radio/aidl/compat/libradiocompat/modem/RadioModem.cpp
index 660ae9f..3fff5ee 100644
--- a/radio/aidl/compat/libradiocompat/modem/RadioModem.cpp
+++ b/radio/aidl/compat/libradiocompat/modem/RadioModem.cpp
@@ -27,6 +27,10 @@
 namespace aidl = ::aidl::android::hardware::radio::modem;
 constexpr auto ok = &ScopedAStatus::ok;
 
+std::shared_ptr<aidl::IRadioModemResponse> RadioModem::respond() {
+    return mRadioResponse->modemCb();
+}
+
 ScopedAStatus RadioModem::enableModem(int32_t serial, bool on) {
     LOG_CALL << serial;
     mHal1_5->enableModem(serial, on);