Don't set AccessPoint with WifiInfo.INVALID_RSSI

This stops the RSSI of an AccessPoint object being updated from WifiInfo
for the current network, when that WifiInfo does not have a valid RSSI.
This occurs when attempting to connect to a network, and the connection
has not yet been established.
Fixes a UI bug where the signal strength would appear to be zero when
attempting to connect to something, or after failing to.

Bug: 63487286
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java
Test: Manual. Attempt to connect to network with bad PSK. Observe Signal
strength does not get set to zero.

Change-Id: Ia5343fb6f0935134e6a4f82034de164342fe55d4