Separate OTA data usage from SYSTEM_UID.

Prior to this CL, Process.OTA_UPDATE_UID was included with SYSTEM_UID
for the purposes of per-app data usage in the DataUsageList. With this
change, it is now separated into its own category.

This change also deletes the duplicate ic_system_update* icons and
moves the one implementation into SettingsLib.

Fixes: 70640191
Test: manually verified locally that there is an item in the list
with the proper title. Required setting up a custom OTA for my
test device.

Change-Id: Iecc45648f68a12c9f8e2370577ab017863e7fcb2