libbinder_ndk: do not rely on copyability of parcelables

AParcel_readNullableParcelable accidentally relies on T's copyability
(introduced by 3b31ccac.)

This prevents non-copyable parcelables from being used in a nullable
arrays. For example, ScopedFileDescriptor.

The following example should be valid.

  parcelable Foo {
    parcelable Bar { ParcelFileDescriptor pfd; }
    @nullable Bar[] bars;
  }

This fixes it by using std::optional<T>::emplace().

Bug: 203483658
Test: m; build NDK with the example above.
Change-Id: If95b28869feca2114922c117b7eb618af434a535