Add Settings.Global.WIFI_WAKEUP_ENABLED flag.

Set the flag default to false. This addition is for the
Android Wi-Fi Snooze project:
https://docs.google.com/document/d/1zewDZ6Ne-qH7zKFDtcSx55f9jPauZwT5VvOs1uVNXf8/edit#

Bug: 32920000
Bug: 32913119
Test: runtest --path
frameworks/base/packages/SettingsProvider/test/src/com/android/providers/settings/SettingsProviderTest.java

Change-Id: I597d5dd643e8044315cc6d7553ae5740485a0552