Add basic support for remounting ext4 userdata into checkpoint

Metadata encrypted ext4 userdata and v2 encryption keys will be
supported in follow up CLs.

Test: adb shell /system/bin/vdc checkpoint startCheckpoint 1
Test: adb reboot userspace
Bug: 135984674
Bug: 143970043
Change-Id: I8dbf3bddd811cf54d3f2b2ee2c1ea64159d8c6e0