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/ndkstubgen/__init__.py b/cc/ndkstubgen/__init__.py
index 2387e69..5e6b8f5 100755
--- a/cc/ndkstubgen/__init__.py
+++ b/cc/ndkstubgen/__init__.py
@@ -108,7 +108,14 @@
     parser.add_argument(
         '--llndk', action='store_true', help='Use the LLNDK variant.')
     parser.add_argument(
-        '--apex', action='store_true', help='Use the APEX variant.')
+        '--apex',
+        action='store_true',
+        help='Use the APEX variant. Note: equivalent to --system-api.')
+    parser.add_argument(
+        '--system-api',
+        action='store_true',
+        dest='apex',
+        help='Use the SystemAPI variant. Note: equivalent to --apex.')
 
     parser.add_argument('--api-map',
                         type=resolved_path,