Allow pinning of Assistant app

Use bool config_pinnerAssistantApp (config.xml) to enable pinning for Assistant.

Test:  Set config_pinnerAssistantApp to true in config.xml, build & flash, run adb shell dumpsys pinner , check if assistant package is listed as pinned
Bug: 133211801
Bug: 133218284
Change-Id: If2e4c63156d4dd8f63f3318b5e8fb2db6bc4cc8b