Installd: Allow different behavior before bootcomplete

Check dev.bootcomplete in dex2oat(). Use the information for
two changes.

Only switch to the background when we're post bootcomplete.
This will ensure better utilization after upgrades.

Add a second dex2oat-threads property that is used pre
bootcomplete. A separation of these phases allows using less
cores when the device is up, freeing up resources for other
purposes, e.g., avoid jank. The precedence during boot is the
boot property, or the image property if the former doesn't
exist, or the default property as a fallback.

Bug: 23898216
Bug: 24004256
Change-Id: I5063f3fc4b437cbe88c4e94584e01c1c78eccc4d