Improve the implementation of GetAclInterface

The current implementation of GetAclInterface allocates
a new acl_interface_t, initialize it and makes a new copy
upon return, which is unecessary.
This patch improves its implementation by reusing a static variable
and returns a reference to it.

Bug: 263277439
Test: refactoring, existing test pass
Change-Id: Ib6bc5de6a9e84419fd8019be5a7fd16fc0f1368d