Overridden APEX doesn't contribute to the file contexts

This change fixes the problem that when an apex module is overridden by
another override_apex, the <apex_name>-file_contexts are duplicated when
creating the system-level file-contexts.

Fixing this by not emitting the file_context info for the overridden
apex.

In doing so, OverridableModule interface was extended to have
GetOverriddenBy() method which can be used to test whether a module is
an overridden one or not.

Bug: 144338929
Test: m (apex_test amended)
Test: add "override_apex {name:"com.googlge.android.tzdata",
Change-Id: I5e9401c32899bb9987c90cba4185f571dc1a87f0
base:"com.android.tzdata"}" and the build is successful