Allocate isolated UID ranges for app zygote and its children.

Introduce a new range of app-zygote isolated UIDs, [90000..98999].
For each app that uses an application Zygote, allocate a range of
100 isolated UIDs. The application Zygote for an app will get a
UID out of that range, and all other children that are forked
from that zygote will get a UID from the same range.

Bug:  111434506
Test: app Zygote and its children run in the new range of
      isolated UIDs (with SELinux disabled). New set of
      tests for UID allocators pass.

Change-Id: I7a6883a5ddb95683932c93ea77f4e52d8f37fa4f