Support to share Wi-Fi hotspot via QR code

1. QR code scanner (Wi-Fi Easy Connect) does not support sharing Wi-Fi hotspot
   at current stage
2. Wi-Fi hotspot QR code button only shows when Wi-Fi hotspot is enabled
3. The QR code has the security string "WPA" for hotspot's WPA2_PSK

Bug: 123151660
Test: atest WifiTetherSSIDPreferenceControllerTest
            WifiQrCodeTest WifiDppConfiguratorActivityTest
            WifiDppEnrolleeActivityTest
            WifiDppQrCodeGeneratorFragmentTest
            WifiDppQrCodeScannerFragmentTest
            WifiNetworkListFragmentTest
            WifiDppChooseSavedWifiNetworkFragmentTest

Change-Id: I2e89450180b82cc841ee3b15be52bfc6f9f6164d