libsnapshot: fix re-flash after update

If device takes an update from slot A to
B, immediately flashes the B slot, and reboot
into B slot, libsnapshot incorrectly considers
the device booted into the new slot and refuses
to clear update states. Fix this by checking
the UPDATED flag in super partition metadata.

Test: libsnapshot_test
Bug: 143551390
Change-Id: I3cd7bb19b394da6399d4bf2f9d013bfaa7f186f1