Factor out a system_properties interface for interacting with getprop/setprop.

tools/telemetry/telemetry/core/backends/android_rndis.py

Index: tools/telemetry/telemetry/core/backends/android_rndis.py
diff --git a/tools/telemetry/telemetry/core/backends/android_rndis.py b/tools/telemetry/telemetry/core/backends/android_rndis.py
index c2c0e4bbd98a561681ac4442643abdaa9c41c8db..71367469ce7989491557ecc02b5ecc3530a05207 100644
--- a/tools/telemetry/telemetry/core/backends/android_rndis.py
+++ b/tools/telemetry/telemetry/core/backends/android_rndis.py
@@ -113,7 +113,7 @@ class RndisForwarderWithRoot(object):
         ['sudo', 'bash', '-c', 'echo -e "%s" > %s' % (contents, path)])
 
   def _DisableRndis(self):
-    self._adb.RunShellCommand('setprop sys.usb.config adb')
+    self._adb.system_properties['sys.usb.config'] = 'adb'
     self._WaitForDevice()
 
   def _EnableRndis(self):
@@ -326,8 +326,8 @@ doit &
     default_routes = [route[0] for route in routes if route[1] == '00000000']
     return (
       default_routes[0] if default_routes else None,
-      self._adb.RunShellCommand('getprop net.dns1')[0],
-      self._adb.RunShellCommand('getprop net.dns2')[0],
+      self._adb.system_properties['net.dns1'],
+      self._adb.system_properties['net.dns2'],
     )
 
   def _OverrideDns(self, iface, dns1, dns2):
@@ -341,12 +341,11 @@ doit &
       return  # If there is no route, then nobody cares about DNS.
     # DNS proxy in older versions of Android is configured via properties.
     # TODO(szym): run via su -c if necessary.
-    self._adb.RunShellCommand('setprop net.dns1 ' + dns1)
-    self._adb.RunShellCommand('setprop net.dns2 ' + dns2)
-    dnschange = self._adb.RunShellCommand('getprop net.dnschange')[0]
+    self._adb.system_properties['net.dns1'] = dns1
+    self._adb.system_properties['net.dns2'] = dns2
+    dnschange = self._adb.system_properties['net.dnschange']
     if dnschange:
-      self._adb.RunShellCommand('setprop net.dnschange %s' %
-                                (int(dnschange) + 1))
+      self._adb.system_properties['net.dnschange'] = int(dnschange) + 1
     # Since commit 8b47b3601f82f299bb8c135af0639b72b67230e6 to frameworks/base
     # the net.dns1 properties have been replaced with explicit commands for netd
     self._adb.RunShellCommand('ndc netd resolver setifdns %s %s %s' %