Fix volume keys wakeup status handling

The same status flag is used for the three different volume keys,
however nothing prevents users from pressing multiple keys at the
same time. This allows to set the status flag with one volume key
and clear it with the other volume key.

Use one flag per key so that we never end up in an inconsistent
state. This fixes the seldom power button issues that happen when
the "volume wake" feature is enabled.

Change-Id: I08f5f9ff696bef3dd840cff97d570e44ebe03e4e