Separates RSSI poll histogram by channel frequency

Putting RSSI poll for each frequency into a different histogram.
Will be merging the histograms that have similar properties
in the metric aggregation code. (i.e. combine frequencies from 5150 - 5250
into a single histogram because they have similar max power usage)

Bug: 79864669
Test: compile, unit tests
Manual test:
flash
connect to "Google Guest" and wait 15s
Connect to "Boingo GOolge Open" and wait 15s
adb shell dumpsys wifi
verify that histograms are dumped out for 2 different channel
frequencies:
mWifiLogProto.rssiPollCount: Printing counts for [-127, 0]
 {"5540":[{"-57":1},{"-54":1},{"-53":2},{"-52":3},{"-51":1}],"5640":[{"-49":5},{"-48":9},{"-47":3},{"-45":1}]}

Change-Id: I1a4ec8ea23f4641905b5ea82fdc0100c3d55d92e
2 files changed