Allow "cmd package bg-dexopt-job" to take a list of package names

Modify "cmd package bg-dexopt-job" to accept an optional list of
packages to run bg-dexopt on to help with testing.

Test: make && flashall && automated tests (ag/2959616)
Test: adb shell cmd package bg-dexopt-job
Test: adb shell cmd package bg-dexopt-job pkg_name

BUG: 64807719
Change-Id: Ibf1fda88810dd4ec19fbbfe3322c35b201727707