raise contrast for call buttons, optimize paddings/margins

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

app/src/main/res/layout/call_started_message.xml

@@ -32,7 +32,7 @@
     <LinearLayout
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:layout_margin="@dimen/standard_margin"
+        android:layout_marginBottom="@dimen/standard_margin"
         android:orientation="horizontal"
         android:gravity="center">
 
@@ -65,10 +65,9 @@
             android:backgroundTint="@color/nc_darkGreen"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:layout_marginHorizontal="@dimen/standard_margin"
+            android:layout_marginEnd="@dimen/standard_margin"
             app:icon="@drawable/ic_videocam_grey_600_24dp"
             app:iconTint="@color/white"
-            android:alpha="0.8"
             android:textColor="@color/white"
             android:text="@string/video_call" />
 
@@ -78,8 +77,6 @@
             android:backgroundTint="@color/nc_darkGreen"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:layout_marginHorizontal="@dimen/standard_margin"
-            android:alpha="0.8"
             app:icon="@drawable/ic_phone"
             app:iconTint="@color/white"
             android:text="@string/audio_call"