WifiNetworkFactory: Handle screen state changes

Screen state changes are only processed while we're scanning for
matching networks (for an active request).
a) when the screen is off, pause periodic scanning.
b) when the screen is on, resume periodic scanning.

Bug: 117980480
Test: ./frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Change-Id: I4c759380e3b5af24e3f6b2cdb2336aed71c03bd5