android_media_tv_Tuner: Initial member

We initial the 'mDvrMQEventFlag' member in the constructor, so
that if we end up invoking the destructor with having called
android_media_tv_Tuner_configure_dvr() first, we won't use
uninitialized memory as a pointer.

Test: TreeHugger
Change-Id: Iea3efd45c5d68a90ec36ebf95f00cb1e0a0b054d