Adding aconfig and changeID for user-agent reduction

Early steps adding the changeID and Aconfig for user-agent
reduction to help us enable it for Android Baklava+.

Bug: 371034303
Test: not avaible for test right now
Flag: android.webkit.user_agent_reduction

Change-Id: I2f7ea8ba443b41ec88d41a6ffb734ffdcd886514