Extended Power Menu: Initialize reboot reason to the first reason

Without doing this, the string "null" will be sent to the reboot command
and can cause the reboot recovery pre-command to get confused on devices
that use it. (On Galaxy S, it will default into booting into recovery)

Change-Id: Id94863d56c45ff10d3ded7f62e3f2ef7b8cd0e69

core/java/com/android/internal/app/ShutdownThread.java

@@ -147,6 +147,9 @@ public boolean onKey (DialogInterface dialog, int keyCode, KeyEvent event) {
                                 return true;
                             }
                         });
+                // Initialize to the first reason
+                String actions[] = context.getResources().getStringArray(com.android.internal.R.array.shutdown_reboot_actions);
+                mRebootReason = actions[0];
             } else {
                 dialog = new AlertDialog.Builder(context)
                         .setIcon(android.R.drawable.ic_dialog_alert)