Fix crash on missing header colors

An off-by-one error caused invalid colour info to sometimes be generated
if no header colour was set. Under normal conditions, a header colour
should always be set, but due to some bugs in the past in some cases
they may not be set.

Sadly this does change the card colours for cards which are not properly
initialized. But that's better than a crash at least.

app/src/main/java/protect/card_locker/LetterBitmap.java

@@ -22,11 +22,6 @@ import androidx.core.graphics.PaintCompat;
  * is shown instead.
  */
 class LetterBitmap {
-
-    /**
-     * The number of available tile colors
-     */
-    private static final int NUM_OF_TILE_COLORS = 8;
     /**
      * The letter bitmap
      */
@@ -121,7 +116,7 @@ class LetterBitmap {
     private static int pickColor(String key, TypedArray colors) {
         // String.hashCode() is not supposed to change across java versions, so
         // this should guarantee the same key always maps to the same color
-        final int color = Math.abs(key.hashCode()) % NUM_OF_TILE_COLORS;
+        final int color = Math.abs(key.hashCode()) % colors.length();
         return colors.getColor(color, Color.BLACK);
     }