Implemented visual state detection functionality for revent workloads

- Added statedetect.py in utils which is a standalone module that
    contains all the methods needed for state detection

  - Modified the setup() and run() methods of the GameWorkload class
    in common/android/workload.py to have a parameter that enables
    state checks and run the check after setup and run if requested.

State detection uses the template matching method available in
OpenCV to determine the state of the workload by detecting
predefined unique elements on a screenshot from the device.

doc/source/revent.rst

@@ -210,3 +210,100 @@ https://www.kernel.org/doc/Documentation/input/event-codes.txt
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Event Value                          |
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+
+
+Using state detection with revent
+=================================
+
+State detection can be used to verify that a workload is executing as expected.
+This utility, if enabled, and if state definitions are available for the
+particular workload, takes a screenshot after the setup and the run revent
+sequence, matches the screenshot to a state and compares with the expected
+state. A WorkloadError is raised if an unexpected state is encountered.
+
+To enable state detection, make sure a valid state definition file and
+templates exist for your workload and set the check_states parameter to True.
+
+State definition directory
+--------------------------
+
+State and phase definitions should be placed in a directory of the following
+structure inside the dependencies directory of each workload (along with
+revent files etc):
+
+   dependencies/
+      <workload_name>/
+         state_definitions/
+            definition.yaml
+            templates/
+               <oneTemplate>.png
+               <anotherTemplate>.png
+               ...
+
+definition.yaml file
+--------------------
+
+This defines each state of the workload and lists which templates are expected
+to be found and how many are required to be detected for a conclusive match. It
+also defines the expected state in each workload phase where a state detection
+is run (currently those are setupComplete and runComplete).
+
+Templates are picture elements to be matched in a screenshot. Each template
+mentioned in the definition file should be placed as a file with the same name
+and a .png extension inside the templates folder. Creating template png files
+is as simple as taking a screenshot of the workload in a given state, cropping
+out the relevant templates (eg. a button, label or other unique element that is
+present in that state) and storing them in PNG format.
+
+Please see the definition file for Angry Birds below as an example to
+understand the format. Note that more than just two states (for the afterSetup
+and afterRun phase) can be defined and this helps track the cause of errors in
+case an unexpected state is encountered.
+
+.. code-block:: python
+
+    workload_name: angrybirds
+
+    workload_states:
+      - state_name: titleScreen
+        templates:
+          - play_button
+          - logo
+        matches: 2
+      - state_name: worldSelection
+        templates:
+          - first_world_thumb
+          - second_world_thumb
+          - third_world_thumb
+          - fourth_world_thumb
+        matches: 3
+      - state_name: level_selection
+        templates:
+          - locked_level
+          - first_level
+        matches: 2
+      - state_name: gameplay
+        templates:
+          - pause_button
+          - score_label_text
+        matches: 2
+      - state_name: pause_screen
+        templates:
+          - replay_button
+          - menu_button
+          - resume_button
+          - help_button
+        matches: 4
+      - state_name: level_cleared_screen
+        templates:
+          - level_cleared_text
+          - menu_button
+          - replay_button
+          - fast_forward_button
+        matches: 4
+
+    workload_phases:
+      - phase_name: setup_complete
+        expected_state: gameplay
+      - phase_name: run_complete
+        expected_state: level_cleared_screen