Use explicit requirements from APEX

Previously, CAS APEX used ":sphal" and `android_load_sphal_library` to
open plugins from /vendor/lib/mediacas. It was a workaround because
there's no way to limit the scope to /vendor/lib/mediacas. And CAS
plugins are not SPHAL libraries.

Now, the APEX uses more explicit requirements: ":mediacas", which
limitss the scope to /vendor/lib/mediacas.

Bug: 298542407
Test: atest CtsMediaMiscTestCases:MediaCasTest
Change-Id: Id91a87e0f6ef8a761e37cabed7da8b9fed0591c2