Prevent double-closing of native handle in HidlMemory

Since the handle wasn't nullified, when the memory is explicitly
closed, the finalizer would attempt to close again, which results in
an exception.

Bug: 189213084
Test: Manual verification of ST use-cases, which uses this.
      Patch sent to partner for verification.
Change-Id: I4ac6bbdd88b3f729a022896adfed00effec48bd8