improve icon fitting to theme

src/main/java/eu/siacs/conversations/ui/util/ConversationMenuConfigurator.java

@@ -128,9 +128,9 @@ public class ConversationMenuConfigurator {
         if (!visible) {
             return;
         }
-
+        boolean darkBackground = activity.isDarkTheme();
         if (conversation.getNextEncryption() != Message.ENCRYPTION_NONE) {
-            menuSecure.setIcon(R.drawable.ic_lock_black_24dp);  //TODO: Add white icon for dark mode too!!
+            menuSecure.setIcon(darkBackground ? R.drawable.ic_lock_white_24dp : R.drawable.ic_lock_black_24dp);  //TODO: Add white icon for dark mode too!!
         }
         otr.setVisible(Config.supportOtr() && activity.enableOTR());
         if (conversation.getMode() == Conversation.MODE_MULTI) {

src/main/res/drawable/ic_arrow_back_white_24dp.xml

@@ -0,0 +1,6 @@
+<vector android:alpha="0.77" android:autoMirrored="true"
+    android:height="24dp" android:tint="#FFFFFF"
+    android:viewportHeight="24" android:viewportWidth="24"
+    android:width="24dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="@android:color/white" android:pathData="M16.62,2.99c-0.49,-0.49 -1.28,-0.49 -1.77,0L6.54,11.3c-0.39,0.39 -0.39,1.02 0,1.41l8.31,8.31c0.49,0.49 1.28,0.49 1.77,0s0.49,-1.28 0,-1.77L9.38,12l7.25,-7.25c0.48,-0.48 0.48,-1.28 -0.01,-1.76z"/>
+</vector>