Avoid adding VINTF manifest fragment more than once.

In the case of building a multilib library that defines LOCAL_VINTF_FRAGMENTS,
the previous code would create duplicate rules for the fragment. This change
uses the my_vintf_new_installed variable (which may be empty) to avoid the
duplication.

Bug: 139268165
Test: Verify that build warnings are gone, yet VINTF fragment still installed.
Change-Id: I579ec650019a29a6a105118ad5e5ae9376e8d9bc