Add temporary packageName-based whitelist for allowing

background activity launches

Necessary for early stages of enabling the feature in
enforcing mode - we'll need to temporarily whitelist
apps like Duo, so dogfooders can still use them.

Bug: 123354556
Test: atest WmTests:ActivityStarterTests
Test: adb shell settings put global background_activity_starts_package_names_whitelist com.ejc.bbc:com.whatever.dude
Change-Id: I2515b8ba334e87e7f201569ba5a6bb1b79395354