Initial support for application Zygote.

When an application has requested isolated services to be spawned
from an app zygote, we need to spawn the app zygote itself, and then
ask it to fork an isolated service.

The application zygote currently only creates the class loader, and
doesn't do much else. We keep track of the isolated services that
use the app zygote, and when the last isolated service goes away,
we stop the app zygote itself (after a timeout).

The app zygote itself runs with the app's UID and under the app
seccomp filter. That last one is too restricted, so this currently
only works with SELinux disabled.

Future CLs will add an application callback for preloading.

Test: start multiple isolated services with useAppZygote="true",
      verify app_zygote starts, services start as a child of
      app_zygote. Stopping all services stops app_zygote as well.

Bug: 111434506

Change-Id: I10ee1d4bd148c9298974d434fbc5e5eccbec16cb