init AndroidCommand after the device online to root it

AndroidCommand object will root the device when it is created. 
If this happened before the device is online, error will appear.

BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10695170

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@146306 0039d316-1c4b-4281-b951-d872f2087c98

build/android/emulator.py

@@ -196,7 +196,6 @@ class Emulator(object):
     After confirming a wait-for-device can be successful, make sure
     it returns the right answer.
     """
-    a = android_commands.AndroidCommands(self.device)
     seconds_waited = 0
     number_of_waits = 2  # Make sure we can wfd twice
     adb_cmd = "adb -s %s %s" % (self.device, 'wait-for-device')
@@ -221,7 +220,7 @@ class Emulator(object):
     if wait_for_boot:
       # Now that we checked for obvious problems, wait for a boot complete.
       # Waiting for the package manager is sometimes problematic.
-      a.Adb().SetTargetSerial(self.device)
+      a = android_commands.AndroidCommands(self.device)
       a.Adb().WaitForSystemBootCompleted(self._WAITFORBOOT_TIMEOUT)
 
   def Shutdown(self):