Remove deviceInstalls from bootImageVariant

The use of this field to return information from buildBootImageVariant
up the call stack to one of the callers resulted in data races being
detected. This change simply passes the deviceInstalls up the call
stack.

Bug: 245956352
Test: m nothing
      go test -race ./sdk/... -run TestSnapshotWithBootclasspathFragment_ImageName -test.count 100
      # Run the previous command without this change and sometimes it
      # shows the data race around deviceInstalls. When run with this
      # change it reports no data races.
Change-Id: I3c73920dcb17a6c89a63c6a9c3a0bb049a98a690