Added WifiSettings Espresso tests.

Added test cases for toggling Wi-Fi state, connected access points, and
saved network preferences.

Bug: 35322629
Bug: 35322690
Bug: 35322875
Test: runtest --path
packages/apps/Settings/tests/unit/src/com/android/settings/wifi/WifiSettingsUiTest.java

Change-Id: Iaba7fb2fa90cf0f37538c4b1ef141cf6b15d7f52