Don't unmount APEXes in microdroid

In microdroid, APEXes are passed as block devices from host, and then
mounted directly, without using /data. Unmounting APEXes on reboot is
for clean unmount of /data, so it's not needed in microdroid.

This skips unmounting in microdroid to remove unnecessary SELinux
denials.

Bug: 256793087
Test: run microdroid and reboot, see no denials happen
Change-Id: Iaf737876f5ab5de5e8f76ed8bb0963cc25d73a80