Make tab text and indicator themeable

OpenKeychain/src/main/res/layout/toolbar_tabs.xml

@@ -17,8 +17,8 @@
         android:layout_width="match_parent"
         android:layout_height="?attr/actionBarSize"
         android:background="?attr/colorPrimary"
-        android:textColor="@color/tab_text"
-        app:pstsTextColorSelected="@color/tab_text_selected"
-        app:pstsIndicatorColor="@color/tab_indicator" />
+        android:textColor="?attr/colorTabText"
+        app:pstsTextColorSelected="?attr/colorTabTextSelected"
+        app:pstsIndicatorColor="?attr/colorTabIndicator" />
 
 </RelativeLayout>

OpenKeychain/src/main/res/layout/view_key_activity.xml

@@ -75,7 +75,7 @@
             android:layout_marginEnd="48dp"
             android:text=""
             tools:text="My Key"
-            android:textColor="@color/tab_text"
+            android:textColor="?attr/colorTabText"
             android:textAppearance="?android:attr/textAppearanceSmall"
             android:layout_above="@+id/toolbar2" />