Introduce EVENT_POLL_SIGNAL_STRENGTH_DONE in SignalStrengthController

EVENT_POLL_SIGNAL_STRENGTH_DONE is just a suger event type which
fall through to EVENT_GET_SIGNAL_STRENGTH which was used to
handle the finishing of signal strength polling. This is helpful
for readability with almost no cost.

Bug: 178429976
Test: atest SignalStrengthControllerTest
Change-Id: I56639a66ce82d40f137cd212e8b2c4d8f7806f2e