init: CanMountApexBeforeData()

init sets up mount namespaces for apexd. Whether it needs to create two
mount namespaces depends on whether apexd can mount apexes before data.

For the first boot after factory reset, since there's no data apexes,
init can decide by looking up "apexd.config.compressed_apex". If there's
no compressed apexes, apexd should be able to mount apexes before data.

Otherwise, apexd had a chance to decide whether it can mount apexes
before data partition. Hence, init just looks up
/metadata/apex/config/mount_before_data which is created by apexd.

Bug: 381175707
Flag: com.android.apexd.flags.mount_before_data
Test: flash with data wiped out
Test: device boots with mount_before_data enabled
Test: reboot & device still boots with mount_before_data enabled
Change-Id: If9d03fc2a8910a9315fea74216bf73f6302e6027