Add getFrequency() to WifiInfoTest (along with other missing getters),
and test that it is consistent with ScanResult in ScanResultTest.
Test: atest android.net.wifi.cts.ScanResultTest
Fixes: b/138929469
Merged-In: I070b16661bc72a5c5035b0b227821b680d7d71ba
(cherry picked from commit 2d504956e63eefb0a6a90844d1d00b99c097891b)
Change-Id: Ie99011acbbe66e9088f73964fd0c39d640594011