userdata_checkpoint_remount: delete all devices in dm-stack under /data

This enables unmounting + remounting of metadata encrypted ext4
userdata. Also added a sysprop to force unmount+remount logic even for
f2fs userdata which comes handy for testing, since we don't have
metadata encrypted ext4 devices to test on.

Test: adb shell setprop sys.init.userdata_remount.force_umount_f2fs 1
Test: adb shell /system/bin/vdc checkpoint startCheckpoint 1
Test: adb reboot userspace
Test: adb shell dumpsys activity
Bug: 135984674
Bug: 143970043
Change-Id: Icc30f5c99f4d9ee374352610ae17b5d9814f815b