libsnapshot: Eliminate per-snapshot flocks.

Per-snapshot locks don't solve any problems and add a great deal of
complexity. Instead, refactor the Read/WriteSnapshotStatus methods so
the caller just needs the snapshot name, and is not responsible for
opening a file.

As part of this change, callers of WriteSnapshotStatus must always take
an exclusive flock on the update state file. This is enforced by adding
a helper method to LockedFile to check the lock mode.

Bug: 136678799
Test: libsnapshot_test gtest
Change-Id: Icd580aaec7dfc916b3eed174d86b26688cd2291b