Create RemoteImage RemoteInstance classes.

- Add kernel_build_id in avd_spec for cuttlefish remote instance.
- handling AVD config the legacy way (cfg) and the new way (AVDSpec)
- adding new metadata to the gce instance to enable other acloud features

Bug: 114166856
Test: m acloud && acloud create
atest acloud_test
Change-Id: Id9ec431e84a6311db9ba09d1fa3ee102452d4957
12 files changed