HW Key Actions: better handling of BACK key
Some apps (e.g. Google Now) override default BACK key behaviour
and perform back action immediately on DOWN event. This is undesired
especially in case we use "Kill foreground app" action since foreground
app goes away on BACK key DOWN; long-press action is then triggered
within another foreground app.
To make this work, we have to completely intercept BACK key behaviour
when HW key action is assigned. This approach requires additional BACK
key injection in case we did not perform any action (e.g. user kept key
pressed for shorter time and long-press trigger didn't go off). Thanks
to FLAG_FROM_SYSTEM we can distinguish between actions that were sent
either from actual key (hardware or virtual) or were synthetically
invoked by injecting.