Replace #apex with #systemapi

The intended use is
- #apex is APEX-visible symbols provided by apex libraries
- #systemapi is APEX-visible symbols provided by platform libraries

Since libandroid is a platform library, update the annotation to
sytemapi.

Bug: 239274367
Bug: 265485106
Test: TH
Change-Id: I28db5cd0d3a76afa1a7ecac042ee02dd74faa2f1