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
diff --git a/cc/symbolfile/__init__.py b/cc/symbolfile/__init__.py
index 9813467..31c4443 100644
--- a/cc/symbolfile/__init__.py
+++ b/cc/symbolfile/__init__.py
@@ -83,7 +83,7 @@
     @property
     def has_apex_tags(self) -> bool:
         """Returns True if any APEX tags are set."""
-        return 'apex' in self.tags
+        return 'apex' in self.tags or 'systemapi' in self.tags
 
     @property
     def has_llndk_tags(self) -> bool: