Add Launcher aconfig.

Use `enable_expanding_pause_work_button` as an example.

See go/trunk-stable-launcher for implementation details

Bug: 294913042

Test: adb shell device_config put launcher com.google.android.platform.launcher.aconfig.flags.enable_expanding_pause_work_button true

Flag: enable_expanding_pause_work_button
Change-Id: I732722c8b219c023adf5bf31f132ce9da72fc4d5