libsnapshot: Add EnsureMetadataMounted

In recovery, client is responsible for calling
EnsureMetadataMounted before doing (almost) all
operations on SnapshotManager, e.g.

- CancelUpdate() before sideloading
- BeginUpdate() on retrofit Virtual A/B before sideloading
- Finishing merge before flashing

Test: libsnapshot_test
Test: recovery sideload
Bug: 140749209
Change-Id: I1034a7fa74e31b6850896e61e86341239dbf2699