Optimize gesture preview trail drawing a bit

This change also change the fading out duration of the gesture
preview trail to 800msec from 1,000msec

Change-Id: Icfd5b1ecad66db4f978249486f5fe362a3ae9640
diff --git a/java/res/values/config.xml b/java/res/values/config.xml
index 8477df0..8e2d43e 100644
--- a/java/res/values/config.xml
+++ b/java/res/values/config.xml
@@ -51,7 +51,7 @@
     <integer name="config_key_preview_linger_timeout">70</integer>
     <integer name="config_gesture_floating_preview_text_linger_timeout">200</integer>
     <integer name="config_gesture_preview_trail_fadeout_start_delay">100</integer>
-    <integer name="config_gesture_preview_trail_fadeout_duration">1000</integer>
+    <integer name="config_gesture_preview_trail_fadeout_duration">800</integer>
     <integer name="config_gesture_preview_trail_update_interval">20</integer>
     <!--
          Configuration for MainKeyboardView