qdutils: idle_invalidator: Use fixed timeout rather than range

Currently the idle timeout is in a range from idle_timeout / 2
to idle_timeout, since we didn't need precise values.

Change this to a fixed value using timestamps, because the timeout
is too low and warrants precision.

Change-Id: I9cec1db7f1d7fda2996ab80449c61269915f6be4
diff --git a/libqdutils/idle_invalidator.h b/libqdutils/idle_invalidator.h
index abd9b29..f41c15e 100644
--- a/libqdutils/idle_invalidator.h
+++ b/libqdutils/idle_invalidator.h
@@ -38,6 +38,7 @@
 
 class IdleInvalidator : public android::Thread {
     void *mHwcContext;
+    struct timeval mLastUpdateTime;
     bool mSleepAgain;
     unsigned int mSleepTime;
     static InvalidatorHandler mHandler;
@@ -49,7 +50,7 @@
     /* init timer obj */
     int init(InvalidatorHandler reg_handler, void* user_data, unsigned int
              idleSleepTime);
-    void markForSleep();
+    void handleUpdateEvent();
     /*Overrides*/
     virtual bool        threadLoop();
     virtual int         readyToRun();