Do not expose device target-levels in trout manifests

Cuttlefish already specifies a manifest level, and specifying it
in trout can break if there's any inconsistency with what Cuttlefish
is outlining.

Not specifying an explicit level will silently pick up the default
used by our upstream cf targets.

Bug: 164120782
Bug: 163653133
Test: build
Change-Id: I82e1368e01c42af0e72dca6751baebd230fc75ab
5 files changed