[Android] Add GetDescription and defer calculating device_utc_offset

First, only get the device's utc offset when it is actually needed. Calculating
this offset takes about 100ms on a typical device. The build scripts assume
that creating an instance of AndroidCommands is not so expensive. Instead,
defer calculating this value until it is actually requested. There are
currently several cases where the build scripts call adb directly instead of
through android_commands just to avoid this extra cost.

Second, add a GetDescription method to get a (mostly) user-readable description
of the device.

TBR=frankf@chromium.org

Review URL: https://chromiumcodereview.appspot.com/17088005

git-svn-id: http://src.chromium.org/svn/trunk/src/build@207371 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/pylib/android_commands.py

@@ -219,7 +219,7 @@ class AndroidCommands(object):
     self.logcat_process = None
     self._logcat_tmpoutfile = None
     self._pushed_files = []
-    self._device_utc_offset = self.RunShellCommand('date +%z')[0]
+    self._device_utc_offset = None
     self._md5sum_build_dir = ''
     self._external_storage = ''
     self._util_wrapper = ''
@@ -848,7 +848,12 @@ class AndroidCommands(object):
                          '(?P<filename>[^\s]+)$')
     return _GetFilesFromRecursiveLsOutput(
         path, self.RunShellCommand('ls -lR %s' % path), re_file,
-        self._device_utc_offset)
+        self.GetUtcOffset())
+
+  def GetUtcOffset(self):
+    if not self._device_utc_offset:
+      self._device_utc_offset = self.RunShellCommand('date +%z')[0]
+    return self._device_utc_offset
 
   def SetJavaAssertsEnabled(self, enable):
     """Sets or removes the device java assertions property.
@@ -898,8 +903,17 @@ class AndroidCommands(object):
     assert build_type
     return build_type
 
+  def GetDescription(self):
+    """Returns the description of the system.
+
+    For example, "yakju-userdebug 4.1 JRN54F 364167 dev-keys".
+    """
+    description = self.RunShellCommand('getprop ro.build.description')[0]
+    assert description
+    return description
+
   def GetProductModel(self):
-    """Returns the namve of the product model (e.g. "Galaxy Nexus") """
+    """Returns the name of the product model (e.g. "Galaxy Nexus") """
     model = self.RunShellCommand('getprop ro.product.model')[0]
     assert model
     return model