Use apexservice to get the list of apexes

In this change, the logic to get the list of apexes is replaced with
the query to apexservice, which is the source of truth. This dedups
the manual scanning of /apex for the apex list, which should be
internal to apexd. (Note how vold_prepare_subdirs filtered out
directories with "@" character).

This also makes vold_prepare_subdirs immune to the upcoming changes:
/apex directory containing only bootstrap APEXes in the bootstrap NS.

Until now, getting the list of apexes by scanning /apex was okay
because of the accidental fact that /apex directory has directories
for all the apexes, not only bootstrap apexes.

Bug: 293949266
Bug: 293546778
Test: CtsPackageSettingHostTestCases
Change-Id: I3fe373ca6f4c2281439bb2449845a1a14357131e