Adding a new config and Setting for WiFi Wakeup.

Created a new config for WiFi Wakeup (config_wifi_wakeup_available)
and defaulted it to NOT_AVAILABLE.

Also added a new global Settings constant (WIFI_WAKEUP_AVAILABLE)
and defaulted it to the value of the new config noted above.

Bug: 37987491
Bug: 38036968
Test: Built, flashed and confirmed proper config value.
Test: runtest --path frameworks/base/core/tests/coretests/src/android/provider/SettingsProviderTest.java
Change-Id: I0cdca4262a0ef473fcbbf7da8d960c41dfafb11f