[CTS] Wait connection stable then start RTT

Test device may have saved networks, turn on Wi-Fi will lead a sequence
of connection operations which may reduce RTT performance.
Wait for connection stable then start RTT test.

Bug: 155218926
Test: atest WifiRttTest
Change-Id: I3ca5cd6efb246ac3e429ef3d8079d803f5eb9408
1 file changed