apexd: use "APEX select" instead of "Multi-install"

APEX Select property can be used even for a single install case now.

Minor refactoring around ApexFileRepository constructor and tests. We
can always check partitions even in tests. So, no need for "enforce"
flag which was used for testing only.

Bug: 417632996
Test: ApexTestCases
Change-Id: I67e2c4d6fd00fcd105ad360e6225e8634b2e0c06
4 files changed