Refer to the last Hotspot password when generating the password

- Query the last Hotspot password when security type is SECURITY_TYPE_OPEN and it will be used when the user changes to another security type

- If the last hotspot password is an empty string, then generate random password

Bug: 201789473
Test: manual test
atest -c WifiHotspotRepositoryTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherPasswordPreferenceControllerTest

Change-Id: Id3a846099238433e7b6c8b934fa26a74df0dcb61

tests/spa_unit/src/com/android/settings/testutils/FakeFeatureFactory.kt

@@ -46,6 +46,7 @@ import com.android.settings.slices.SlicesFeatureProvider
 import com.android.settings.users.UserFeatureProvider
 import com.android.settings.vpn2.AdvancedVpnFeatureProvider
 import com.android.settings.wifi.WifiTrackerLibProvider
+import com.android.settings.wifi.factory.WifiFeatureProvider;
 import com.android.settingslib.core.instrumentation.MetricsFeatureProvider
 import org.mockito.Mockito.mock
 
@@ -177,4 +178,8 @@ class FakeFeatureFactory : FeatureFactory() {
     override fun getAdvancedVpnFeatureProvider(): AdvancedVpnFeatureProvider {
         TODO("Not yet implemented")
     }
+
+    override fun getWifiFeatureProvider(): WifiFeatureProvider {
+        TODO("Not yet implemented")
+    }
 }