Disable webrtc calling for Gingerbread users

Fixes #6220
// FREEBIE

src/org/thoughtcrime/securesms/ApplicationContext.java

@@ -18,6 +18,7 @@ package org.thoughtcrime.securesms;
 
 import android.content.Context;
 import android.os.AsyncTask;
+import android.os.Build;
 import android.os.StrictMode;
 import android.os.StrictMode.ThreadPolicy;
 import android.os.StrictMode.VmPolicy;
@@ -87,8 +88,10 @@ public class ApplicationContext extends MultiDexApplication implements Dependenc
     initializePeriodicTasks();
     initializeCircumvention();
 
+    if (Build.VERSION.SDK_INT >= 11) {
       PeerConnectionFactory.initializeAndroidGlobals(this, true, true, true);
     }
+  }
 
   @Override
   public void injectDependencies(Object object) {

src/org/thoughtcrime/securesms/preferences/AdvancedPreferenceFragment.java

@@ -6,6 +6,7 @@ import android.content.DialogInterface;
 import android.content.Intent;
 import android.content.pm.PackageManager;
 import android.net.Uri;
+import android.os.Build;
 import android.os.Bundle;
 import android.preference.CheckBoxPreference;
 import android.preference.Preference;
@@ -98,8 +99,13 @@ public class AdvancedPreferenceFragment extends PreferenceFragment {
   }
 
   private void initializeWebrtcCallingToggle() {
+    if (Build.VERSION.SDK_INT >= 11) {
       this.findPreference(TextSecurePreferences.WEBRTC_CALLING_PREF)
           .setOnPreferenceChangeListener(new WebRtcClickListener());
+    } else {
+      this.findPreference(TextSecurePreferences.WEBRTC_CALLING_PREF)
+          .setEnabled(false);
+    }
   }
 
   private void initializeIdentitySelection() {