generate sender_id const from ENV so doesn't need to be checked in.

Now works to register as I'm using the right id.

xwords4/android/XWords4-gcm/build.xml

@@ -79,6 +79,9 @@
             <arg value="--dest-dir"/>
             <arg value="./assets"/>
         </exec>
+        <exec dir="." executable="../scripts/gen_gcmid.sh"
+              output="src/org/eehouse/android/xw4gcm/GCMConsts.java"
+              />
         <exec dir="." executable="../scripts/ndkbuild.sh" output="/dev/null">
             <arg value="-j3"/>
         </exec>

xwords4/android/scripts/gen_gcmid.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+if [ -z "$GCM_SENDER_ID" ]; then
+    echo "GCM_SENDER_ID not in env"
+    exit 1
+fi
+
+cat <<EOF
+// Auto-generated: DO NOT CHECK THIS IN until questions about
+// obscuring the id are cleared up
+package org.eehouse.android.xw4gcm;
+
+public class GCMConsts {
+    public static final String SENDER_ID = "${GCM_SENDER_ID}";
+}
+EOF