Consolidate getRssi logic to return mRssi.

Changed previous getRssi method to updateeRssi in order to clarify
process, and also changed logic to return current mRssi when the given
AP is active. mRssi is set directly from WifiInfo for active networks.
For non-active networks we return the highest Rssi for any recently seen
BSSID (multiple APs with the same SSID are grouped together).

Change getSeen to updateSeen for consistency, although seen is never
used anywhere.

Also fixes b/34889252.

Bug: b/36077865
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: Ic52df41b8eb317e83d835c745b404007a53cf5d7