Add a metadata folder for libsnapshot.

/metadata/ota will store the update state ("none", "applying",
"booting", "merging") for each dynamic partition. The data will be
managed by libsnapshot, whose primary consumer will be update_engine
but will also be available to recovery/fastbootd.

Bug: 136678799
Test: /metadata/ota exists
Change-Id: I3e06484cafeb363904914767abc8984adaa37021