Add systemapi as an APEX synonym for stub maps.

The apex tag is currently used for platform-to-APEX, APEX-to-APEX, and
APEX-to-platform, and it's difficult to tell when reviewing the map
files. Add a synonym so authors can be explicit about platform-to-APEX
since those have different stability rules.

Test: pytest
Test: mypy
Test: pylint
Test: treehugger
Bug: None
Change-Id: I0aee679a05b1b2d3749307434c19486bf4c7fe52