VNDK APEX should not provide native libs

While libz has stubs for other mainline modules, it doesn't provide
stubs when it is used as a VNDK lib.

In general, VNDK libs are only for vendor modules. So, we skip them even
if they have stubs.

Bug: 155456180
Test: m com.android.vndk.current
      provideNativeLibs of the APEX should be empty
Change-Id: I22401a2b1732a8560802c5be850181682e183a8e