Improve readability of power monitoring code and add additional

tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py

Index: tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py
diff --git a/tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py b/tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py
index 8d43be5c1b70d83edc2b7c7d0fffd0729506809e..1802937723949c1253a2e32ba24ffc790120a494 100644
--- a/tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py
+++ b/tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py
@@ -46,7 +46,6 @@ class SysfsPowerMonitor(power_monitor.PowerMonitor):
     self._initial_freq = None
     self._platform = linux_based_platform_backend
     self._standalone = standalone
-    self._is_monitoring_power = False
 
   @decorators.Cache
   def CanMonitorPower(self):
@@ -54,18 +53,16 @@ class SysfsPowerMonitor(power_monitor.PowerMonitor):
         'if [ -e %s ]; then echo true; fi' % CPU_PATH))
 
   def StartMonitoringPower(self, _browser):
-    # |_browser| is unused, can be None.
-    assert not self._is_monitoring_power, 'Must call StopMonitoringPower().'
+    self._CheckStart()
     if self.CanMonitorPower():
       self._cpus = filter(  # pylint: disable=deprecated-lambda
           lambda x: re.match(r'^cpu[0-9]+', x),
           self._platform.RunCommand('ls %s' % CPU_PATH).split())
       self._initial_freq = self.GetCpuFreq()
       self._initial_cstate = self.GetCpuState()
-      self._is_monitoring_power = True
 
   def StopMonitoringPower(self):
-    assert self._is_monitoring_power, 'StartMonitoringPower() not called.'
+    self._CheckStop()
     try:
       out = {}
       if SysfsPowerMonitor.CanMonitorPower(self):
@@ -84,7 +81,6 @@ class SysfsPowerMonitor(power_monitor.PowerMonitor):
         return self.CombineResults(out, {})
       return out
     finally:
-      self._is_monitoring_power = False
       self._initial_cstate = None
       self._initial_freq = None