Device-mapper devices readable by system

With the feature (flag), downloaded APEX files are exposed as
device-mapper devices by apexd. These files should be accessible by
PackageManager.

Bug: 411876541
Flag: com.android.apex.flags.mount_before_data
Test: adb install <apex> & reboot
Change-Id: I249b1a2142a485bc846a56035b256d007335cfc6