Add APIs for Wifi/Cellular coex channel avoidance

Added SystemApis for getting and setting the unsafe channels to avoid
for framework Wifi/Cellular coex channel avoidance.

Bug: 153651001
Test: atest WifiManagerTest, atest CoexUnsafeChannelTest
Change-Id: I4cb4137766c23f096951a2a5a4df1bd946f6c446