Add PARSE_IS_SYSTEM_DIR when parsing /system APEXes.

To prevent us from reading all APEXes at boot and verifying their
integrity; this is not necessary because /system is protected by
dm-verity.

Bug: 126514108
Bug: 117823094
Test: verified system APEXes are no longer read entirely at boot
Change-Id: I1dcf97ce63505602d2de1913e728a1d57b0e9964