Mark all InputReaderContext methods as pure virtual

Also, fix the build.

Change-Id: I45d3daa8c0710b2f9421e8ffd011d2d39c05e030
diff --git a/services/inputflinger/InputReader.h b/services/inputflinger/InputReader.h
index baff7fa..451a20f 100644
--- a/services/inputflinger/InputReader.h
+++ b/services/inputflinger/InputReader.h
@@ -420,8 +420,8 @@
     virtual void requestTimeoutAtTime(nsecs_t when) = 0;
     virtual int32_t bumpGeneration() = 0;
 
-    virtual void getExternalStylusDevices(Vector<InputDeviceInfo>& outDevices);
-    virtual void dispatchExternalStylusState(const StylusState& outState);
+    virtual void getExternalStylusDevices(Vector<InputDeviceInfo>& outDevices) = 0;
+    virtual void dispatchExternalStylusState(const StylusState& outState) = 0;
 
     virtual InputReaderPolicyInterface* getPolicy() = 0;
     virtual InputListenerInterface* getListener() = 0;