Mandate #apex or #systemapi tag for the Mainline APIs

Currently, tagging a symbol with #apex is not required when the symbol
is in a non-NDK library. However, this is considered dangerous because
such a symbol will automatically be promoted to NDK APIs when the
library is promoted to an NDK library. When that happens, the native API
council won't be able to notice the promotion because promoting a
non-NDK library into an NDK library doesn't require an update of the
map.txt file, but Android.bp only.

To prevent that, we should mandate #apex (or #systemapi if the api is
provided by the system, not apex) tag for Mainline APIs regardless of
whether the library the API belongs to is an NDK library or not.

Bug: 184712170
Test: m
Change-Id: Iad589b606bc6ed06ef1237f4c9f0c22518b0db75