Start supplemental processes in new UID range.

When we start a supplemental process on behalf of an app, we want to
start it in its designated UID range. To do that, modify ServiceRecord
to keep track of which app we started the supplemental process for;
then, modify the various startProcess calls to recognize supplemental
processes and assign the correct UID.

Bug: 215012578
Test: atest SupplementalProcessTests
Change-Id: I6338666eaeb39f8775f38878e1db4221c1a0def0