VTS: Free allocated buffers

The native handle was not freed which causes some devices to
run out of memory when executing the test. This change
creates a RAII wrapper around native_handle_t, which
automatically deallocates buffers when they go out of scope.

Bug: 188686850
Test: atest VtsHalGraphicsComposerV2_4TargetTest
Change-Id: I19a22a5a3202e048de77926ce4a116a31d9de906