add an api for getting a device serial number

Add a constant to android.os.Build that returns a device-specific
alphanumeric serial number.  It's optional -- it may be blank --
though CTS will require it to be present for non-telephony devices.

Change-Id: I338ae5f43ba679a5e98f2f437dc4efdaef0c835d
diff --git a/api/current.xml b/api/current.xml
index 65a8a8e..392ea95 100644
--- a/api/current.xml
+++ b/api/current.xml
@@ -121330,6 +121330,16 @@
  visibility="public"
 >
 </field>
+<field name="SERIAL"
+ type="java.lang.String"
+ transient="false"
+ volatile="false"
+ static="true"
+ final="true"
+ deprecated="not deprecated"
+ visibility="public"
+>
+</field>
 <field name="TAGS"
  type="java.lang.String"
  transient="false"
diff --git a/core/java/android/os/Build.java b/core/java/android/os/Build.java
index fdd3573..8925bd0 100644
--- a/core/java/android/os/Build.java
+++ b/core/java/android/os/Build.java
@@ -62,6 +62,9 @@
     /** The name of the hardware (from the kernel command line or /proc). */
     public static final String HARDWARE = getString("ro.hardware");
 
+    /** A hardware serial number, if available.  Alphanumeric only, case-insensitive. */ 
+    public static final String SERIAL = getString("ro.serialno");
+  
     /** Various version strings. */
     public static class VERSION {
         /**