Add callback path from wpa_supplicant core to
AidlManager::getCertificate.

Order of calls is:
1. eapol_sm_get_certificate (eapol_supp_sm.c)
2. wpa_supplicant_get_certificate_cb (wpas_glue.c)
3. wpas_get_certificate (notify.c)
4. wpas_aidl_get_certificate (aidl.cpp)
5. getCertificate (aidl_manager.cpp)

Bug: 205764502
Test: Manual test - store a key-value pair to
      legacy Keystore. Check that we can retrieve
      the value in supplicant using the new callbacks.
Change-Id: Ibe1fc5a2c50b818101c194bf739632d4c2a302a4
diff --git a/wpa_supplicant/notify.c b/wpa_supplicant/notify.c
index 7e6d042..7ec0f0d 100644
--- a/wpa_supplicant/notify.c
+++ b/wpa_supplicant/notify.c
@@ -1352,3 +1352,8 @@
 
 	wpas_aidl_notify_frequency_changed(wpa_s, frequency);
 }
+
+ssize_t wpas_get_certificate(const char *alias, uint8_t** value)
+{
+	return wpas_aidl_get_certificate(alias, value);
+}