customtabs: Add a Builder class for UI customizations.

Also add customtabs to settings.gradle.

Change-Id: I96be920e25f2439898025b695b75da2e7b133e84
diff --git a/settings.gradle b/settings.gradle
index 85c9ae1..6efb120 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -42,3 +42,6 @@
 
 include ':support-percent'
 project(':support-percent').projectDir = new File(rootDir, 'percent')
+
+include ':support-customtabs'
+project(':support-customtabs').projectDir = new File(rootDir, 'customtabs')