Setup aconfig for communal

This change introduces an aconfig file for communal features, and adds a
first flag which controls the communal hub experience.

Bug: 304584416
Bug: 304585752
Test: verified by adding some local logging code which checks the
communal_hub flag. To turn on and off the flag, I ran: adb shell
device_config put communal com.android.systemui.communal.communal_hub
{true|false}

Change-Id: Ifd9e1f7ca6e103a37d888691a10c1f3d60b7e260