Distinguish # apex and # systemapi symbols

Previously, the symbol tag `# apex` was treated the same as `#
systemapi`. With this CL, they have different meanings.

`# systemapi`: APIs that are defined in the platform (the non-updatable
part), and are exposed to unbundled system components like APEX

`# apex`: APIs that are defined in the APEX and are exposed to the
platform or other APEXes

Bug: 239274367
Test: m
Change-Id: I0484ea349656dbbd337e5fe3a5970f0ad275b807