Add a shell command to force the background dexopt job

Add 'bg-dexopt-job' command to 'adb shell cmd package' which will force
the background  dexopt job to run the optimizations immediately.

Note that the command only runs the background optimizer logic. It may
overlap with the actual job but the job scheduler will not be able to
cancel it. It will also run even if the device is not in the idle
maintenance mode.

Test: adb shell cmd package bg-dexopt-job
Bug: 32871170
Change-Id: I8b0112074220b2e09baf13d842401ee1cf306a89