Re-root and use device remount APIs for SystemPreparer

This might improve success rates for remounting the device.
The command goes through shell with no inherent state, so it's
unclear how it can fail, but going through the official methods
might help.

Bug: 197552347

Test: atest --rerun-until-failure 100 \
    com.android.server.pm.test.OverlayActorVisibilityTest

Change-Id: I25c1f938064e72ddc288b12c048f0cd82217b583