libadfhwc: Fix adf_hwc_close

The adf hwc event thread now responds to pthread_kill which
allows adf_hwc_close to succeed.

Test: Call "adb shell stop" and check if thread exits.
      This change will allow testing of libadfhwc in future patches.

Change-Id: I66d4fc71a058db42af8d42b5d6193378a8b5d972