LineageParts: Add support for customising navbar app switch long press action

*) Also fixes bug where we weren't initializing app switch long press
   setting to config_longPressOnAppSwitchBehavior (even for hardware keys).
   I've verified that the config is set to split screen in lineage-sdk.

Change-Id: I3af7bcf6402cecbb6843d50f7a1000af84e9e53d