Reland of [Android][Telemetry] Add support for multiple power monitors

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 9ebbe088f194141de688783fe21cf5e6ccbe4e3c..528e068a89af20c37f807b2330bd22b66a218466 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
@@ -18,11 +18,14 @@
   """PowerMonitor that relies on sysfs to monitor CPU statistics on several
   different platforms.
   """
-  def __init__(self, linux_based_platform_backend):
+  # TODO(rnephew): crbug.com/513453
+  # Convert all platforms to use standalone power monitors.
+  def __init__(self, linux_based_platform_backend, standalone=False):
     """Constructor.
 
     Args:
         linux_based_platform_backend: A LinuxBasedPlatformBackend object.
+        standalone: If it is not wrapping another monitor, set to True.
 
     Attributes:
         _cpus: A list of the CPUs on the target device.
@@ -42,6 +45,7 @@
     self._initial_cstate = None
     self._initial_freq = None
     self._platform = linux_based_platform_backend
+    self._standalone = standalone
 
   @decorators.Cache
   def CanMonitorPower(self):
@@ -74,6 +78,8 @@
         for cpu in frequencies:
           out[cpu] = {'frequency_percent': frequencies[cpu]}
           out[cpu]['cstate_residency_percent'] = cstates[cpu]
+      if self._standalone:
+        return self.CombineResults(out, {})
       return out
     finally:
       self._initial_cstate = None