libafw: Use std::free explicitly for overlays

Currently we rely on libc to call std::free on the malloced pointer to
the overlay table entry. Rather than relying on the libc implementation
to call std::free instead of trying to delete the pointer as if it was
created using "new", explicitly pass std::free as the deleter to the
data held by the std::shared_ptr.

Bug: 159562360
Test: Observed no heap growth while looping infinitely and retrieving
      a string resource overlaid using an inline xml value
Change-Id: I9a2ebaf9a993ad9c44fab8f052430c8142d4347d