[EGD-6209] Tethering confirmation popup added

Provided a tethering confirmation popup.
The popup shows up once a USB cable is connected to a PC.

[EGD-5834] Fix restore API

Change restore API
backup/restore/update harness tests

[EGD-6101] Add low battery start prevention

Device will be shutdown after
startup if SOC will be below
shutdown level

[EGD-6111] Timers refactored

Deep refactor of system timers.

[EGD-5918] The system should crash on init failure

ServiceDB init process times out. Its timeout should be extended.

[EGD-5304] Add new way of system close

Unified method of closing
the system orchestrated by
system manager

[EGD-5771] Create post update info window

Add post update info window.

[EGD-5836] Turn on/off the tethering via harness

This allows a user to turn on and off the tethering via test harness.

[EGD-5737] Merge master into experimental

Keep experimental up to date with changes from master

[EGD-5737] Merge master into experimental

Keep experimental up to date with changes from master

[EGD-5737] Merge master into experimental

Keep experimental up to date with changes from master

[EGD-5697] Framework for phone modes introduced

It allows to transmit phone modes information to listening services.

[EGD-4068] Add Update Package Installation Mode

It is necessary that the PureOS update mechanism enters Update Package
Installation Mode (where most of the services are stopped)
when the PureOS update package gets downloaded into the phone.

[EGD-5027] Services synchronization introduced

Dependencies between services are now supported.

[EGD-5381] Add Low Power Drivers

Peripherals clock synchronization dependent on PLL2

[EGD-5158] Change access to the bus methods

The bus is now accessible only via Service object.

[EGD-3247] Remove reminders of unused ping-pong feature

Feature was already removed some time ago but still some reminders
persist. This commit only removed mentioned reminders.
Some minor style fixes suggested by clang tidy were also applied.

Merge branch 'master' into stable

[EGD-5325] Remove using namespace directives from headers

"Using namespace" directive should not be added in headers
because this practice pollutes source files.

[EGD-4694] Add CPU frequency shift mechanism

The CPU frequency changes automatically which saves energy