rilmodem: Add powerd atom implementation
plugins: Create powerd atom for mtk and ril modems
unit: Add screen state request tests
build: Add powerd atom and ril implementation
doc: Add powerd atom
test: New script for setting low power mode
mtkmodem: Add fast dormancy support to powerd atom

MTK modems have a separate request to inform the modem of the screen
state for configuring fast dormancy timers.