Fix potential access to invalid memory during shutdown

Since binder threads have a reference to the callback, need to make sure
it is properly reset before deleting the DvrHwcClient object.

Bug: 63139142
Test: Ran on device to ensure VrCore restart don't result in crashes in
DvrHwcClient
Change-Id: I559844a70d4483fee3148526f704d234994a96d4