Ensure dex2oat is run with background priority for OTA dexopt.

This makes us query `dalvik.vm.background-dex2oat-cpu-set` and
`dalvik.vm.background-dex2oat-threads` for those runs.

A test run shows that `DEXOPT_BOOTCOMPLETE` is propagated from
`OtaDexoptService`, but set it anyway to be certain.

Test: Apply OTA and check that `boot_complete` and
      `background_job_compile` get set in `android::installd::dexopt`.
Bug: 237017087
Change-Id: I48627dd062fb6155c826d48b7caefeec36f0cfff