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 libmediandk is a platform library, using # apex annotationn meant
that these symbols were not actually visible to apex variants
(b/265485106)

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