soong: Use the CpExecutable rule for copying libraries

If you compile a shared library with 'cc_library_shared' it will
be executable. If you build one with 'cc_prebuilt_library_shared'
it will not. Use CpExecutable so it's consistent.

Test: Observe that all cc_prebuilt_library_shared modules
      are executable.
Change-Id: I3d7edfc920760aa2f73e4527d4cfab1baa1523fc