userspace reboot: unmount apexes and switch to bootstrap namespace

This will bring device to the state closer to the one during normal boot

Bug: 135984674
Test: adb install system/apex/shim/com.android.apex.cts.shim.v1.apex
Test: adb reboot userspace
Test: verified install succeeded
Change-Id: I6ef73bde2ca817c8a62bf19b8f1895dd0d6d2829