libsnapshot: fix tests

- Add TestDeviceInfo(string, string) to avoid holding
the 'rebooted' TestDeviceInfo variable after ownership is
transferred

- OTA client calls BeginUpdate after unmapping partitions.

Test: run libsnapshot_test
Change-Id: Id37e34b421d728eb320c0e0906abef3b61897e7c
2 files changed