Regenerate odm or vendor using combined sepolicy if --rebuild-sepolicy.

This allows merged devices to boot using a precompiled_sepolicy built
from merged sources, rather than recompiling this sepolicy at boot
time every boot.

Bug: 178727214
Test: Merge an R+S build using --rebuild-sepolicy and --vendor-otatools.
      Observe odm.img is rebuilt by the vendor otatools.zip
        when merging.
      Observe device boots using ODM's precompiled_sepolicy file.
Test: Same as above, for S+S.
Test: Merge an S+S build using --rebuild-sepolicy and *not*
        --vendor-otatools.
      Observe odm.img is rebuilt without using a separate otatools.zip.
      Observe device boots using ODM's precompiled_sepolicy file.
Change-Id: I9595b8a3296d6deec21db8f0c9bc5b7ec4debd57