New flag: hearing_devices_input_routing_control

* Add into SettingsLib to be able to accessed by Settings and SystemUI

Bug: 349255906
Test: 'adb shell device_config list' to check flag exists
Flag: com.android.settingslib.flags.hearing_devices_input_routing_control
Change-Id: I3bc7846a82e5fc3deee53f4a96e77cedd2eff9b7
1 file changed