Add dummy to document its absence

The provided by the upstream fmtlib repository is for use
with the NDK rather than the platform build, so we have replaced it
with an Android.bp. This change adds back a dummy with a
comment documenting this so as to not overly confuse future

Test: None, documentation only
Change-Id: I242a8238acda88c62d3612b33bb59b387efc5885
1 file changed