Lower minSDK version for customtabs

This wass set to 16 before, but there are no APIs
that it uses that requires 16 specifically. Lowering to
15 to avoid issues on the client side implementation.

b/23551257

Change-Id: I0a9e6564554fd55a6e5473a029b81b876c960b1c
diff --git a/customtabs/AndroidManifest.xml b/customtabs/AndroidManifest.xml
index 14332d7..212fab9 100644
--- a/customtabs/AndroidManifest.xml
+++ b/customtabs/AndroidManifest.xml
@@ -15,6 +15,6 @@
 -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
           package="android.support.customtabs">
-    <uses-sdk android:minSdkVersion="16"/>
+    <uses-sdk android:minSdkVersion="15"/>
     <application />
 </manifest>