Allocate new UID for OTA update resource tracking.

Recent changes to OTA updates started "blaming" network usage on the
system UID, which makes it difficult to triage incoming bugreports
that claim heavy network usage.  Instead, this change gives OTA
updates an explicit UID to make triage easier.

Test: builds, boots
Bug: 36130264
Change-Id: I0a0cc009f3d891b19b419bc12cd237ef8ac64519