Update usb charging command to wait for it to succeed.

tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py

Index: tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py
diff --git a/tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py b/tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py
index f96a4777ae680a1b69c92fe4c13994241adc7ee1..5ee5eb1e68c3d863e9e000068b557fec8d75d075 100644
--- a/tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py
+++ b/tools/telemetry/telemetry/core/platform/power_monitor/android_ds2784_power_monitor.py
@@ -28,13 +28,6 @@ class DS2784PowerMonitor(power_monitor.PowerMonitor):
         'file_poller')
 
 
-  def _IsDeviceCharging(self):
-    for line in self._adb.RunShellCommand('dumpsys battery'):
-      if 'powered: ' in line:
-        if 'true' == line.split('powered: ')[1]:
-          return True
-    return False
-
   @decorators.Cache
   def _HasFuelGauge(self):
     return self._adb.FileExistsOnDevice(CHARGE_COUNTER)
@@ -42,7 +35,7 @@ class DS2784PowerMonitor(power_monitor.PowerMonitor):
   def CanMonitorPower(self):
     if not self._HasFuelGauge():
       return False
-    if self._IsDeviceCharging():
+    if self._adb.IsDeviceCharging():
       logging.warning('Can\'t monitor power usage since device is charging.')
       return False
     return True