Update state machine for `Absent` and `Battery Low` states

[denis-stepanov]

Somewhat inspired by issue #16. The mailbox state machine should be updated as follows:

1. state Absent moved to the highest priority, because the de-facto implementation of Absent in the code overrides any other state;
2. Battery Low state is not really a state, because mailbox could be in any other state while having low battery. So this state should be removed and replaced with a dedicated flag.