Create utility method for calculating supplemental_uid from app_uid

Every app will now have a corresponding supplemental process associated
with it. We need an utility method to map one to the other.

Implementation details: supplemental process uid will be between range
20k-30k. As such, it will be a 10k offset from app id. See ag/16621743.

Bug: 211763739
Test: atest installd_service_test
Ignore-AOSP-First: Feature is being developed in internal branch
Change-Id: I2b6d6b086985bcb24c837eaa95a937d429d6a583
Merged-In: I2b6d6b086985bcb24c837eaa95a937d429d6a583
(cherry picked from commit 1c7acfdb)