Cache ITelephony service.

Building on top of ag/10255108 that did the same for some other services
in telephony.

Test: basic telephony validity
Bug: 166314475
Change-Id: Icc3e9db5901655eb1102d5b7be81c26a685aa20d

telephony/java/android/telephony/TelephonyManager.java

@@ -305,6 +305,8 @@ public class TelephonyManager {
     /** @hide */
     private static boolean sServiceHandleCacheEnabled = true;
 
+    @GuardedBy("sCacheLock")
+    private static ITelephony sITelephony;
     @GuardedBy("sCacheLock")
     private static IPhoneSubInfo sIPhoneSubInfo;
     @GuardedBy("sCacheLock")
@@ -5504,8 +5506,34 @@ public class TelephonyManager {
      */
     @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
     private ITelephony getITelephony() {
-        return ITelephony.Stub.asInterface(TelephonyFrameworkInitializer
+        // Keeps cache disabled until test fixes are checked into AOSP.
-                .getTelephonyServiceManager().getTelephonyServiceRegisterer().get());
+        if (!sServiceHandleCacheEnabled) {
+            return ITelephony.Stub.asInterface(
+                    TelephonyFrameworkInitializer
+                            .getTelephonyServiceManager()
+                            .getTelephonyServiceRegisterer()
+                            .get());
+        }
+
+        if (sITelephony == null) {
+            ITelephony temp = ITelephony.Stub.asInterface(
+                    TelephonyFrameworkInitializer
+                            .getTelephonyServiceManager()
+                            .getTelephonyServiceRegisterer()
+                            .get());
+            synchronized (sCacheLock) {
+                if (sITelephony == null && temp != null) {
+                    try {
+                        sITelephony = temp;
+                        sITelephony.asBinder().linkToDeath(sServiceDeath, 0);
+                    } catch (Exception e) {
+                        // something has gone horribly wrong
+                        sITelephony = null;
+                    }
+                }
+            }
+        }
+        return sITelephony;
     }
 
     private IOns getIOns() {
@@ -13401,6 +13429,10 @@ public class TelephonyManager {
     */
     private static void resetServiceCache() {
         synchronized (sCacheLock) {
+            if (sITelephony != null) {
+                sITelephony.asBinder().unlinkToDeath(sServiceDeath, 0);
+                sITelephony = null;
+            }
             if (sISub != null) {
                 sISub.asBinder().unlinkToDeath(sServiceDeath, 0);
                 sISub = null;