Use highlight span instead of background color span

app/src/main/java/eu/faircode/email/HighlightSpan.java

@@ -0,0 +1,18 @@
+package eu.faircode.email;
+
+import android.text.TextPaint;
+import android.text.style.CharacterStyle;
+import android.text.style.UpdateAppearance;
+
+public class HighlightSpan extends CharacterStyle implements UpdateAppearance {
+    private int color;
+
+    public HighlightSpan(int color) {
+        this.color = color;
+    }
+
+    @Override
+    public void updateDrawState(TextPaint tp) {
+        tp.bgColor = color;
+    }
+}