Give password strength indicator colors their own name

Also prune the android color definitions, most of which aren't used.

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

@@ -40,9 +40,9 @@
             android:layout_gravity="end|center_vertical"
             custom:strength="medium"
             custom:showGuides="false"
-            custom:color_fail="@color/android_red_light"
-            custom:color_weak="@color/android_orange_light"
-            custom:color_strong="@color/android_green_light" />
+            custom:color_fail="@color/password_strength_low"
+            custom:color_weak="@color/password_strength_medium"
+            custom:color_strong="@color/password_strength_high" />
 
     </FrameLayout>
 
@@ -57,4 +57,4 @@
         android:ems="10"
         android:layout_gravity="center_horizontal" />
 
-</LinearLayout>
+</LinearLayout>