Move to a callback/plugin architecture

[shankari]

The current sync architecture interface is not clean.

• On both android and iOS, it calls the usercache directly
• On android, it calls the code to fix the state machine automatically, which is in the data-collection plugin.
• On iOS, it is called from the data collection plugin since the sync happens in response to visit changes
• On both, we read the battery level, which is a data collection thing, and uses the battery wrapper.

Need to do a good design and rework this properly.