Implement basic libsnapshot functionality.

This CL implements some of the libsnapshot internals necessary to work
with update_engine. In particular it implements snapshot and update
state, as well as creating and mapping snapshot devices. It does not
implement anything related to merging, nor does it implement the full
update_engine flow.

Update state is stored in /metadata/ota/state. To synchronize callers of
libsnapshot, we always flock() this file at the top of public functions
in SnapshotManager. Internal functions are only called while the lock is
held, and a "LockedFile" guard object is always passed through to
indicate proof-of-lock.

Low-level functions, such as snapshot management, have been moved to
private methods. Higher-level methods designed for update_engine will
ultimately call into these.

This CL also adds some functional tests for SnapshotManager. Test state
is stored in /metadata/ota/test to avoid conflicts with the rest of the
system.

Bug: 136678799
Test: libsnapshot_test gtest
Change-Id: I78c769ed33b307d5214ee386bb13648e35db6cc6