Introduce Global.LOCATION_GLOBAL_KILL_SWITCH

- When it's set to "1", LOCATION_PROVIDERS_ALLOWED will return "".

- When it's updated, the provider will send the changed notification
for LOCATION_PROVIDERS_ALLOWED for all running users.

Bug: 68769804
Test: Manual test with the settings command
Test: atest $ANDROID_BUILD_TOP/frameworks/base/core/tests/coretests/src/android/provider/SettingsBackupTest.java
Change-Id: Ie14ee696788b4152f7ae442da06b0bfa8034db52