free EGL resources associated to a thread when it terminates

destroyed but current-to-a-thread resources are only destroyed
when they're made not-current; however, we were not destroying
those when the thread itself terminated, causing these resources
to be leaked forever.

we now install a tls-key destructor that takes care of this
by calling eglReleaseThread upon thread termination.

Bug: 9209776
Change-Id: I88298a34e3a27488eb81eab76717715569c7d57c