Add support for "empty" server-side flag values

Add support to ServerFlags for a special escape string for empty arrays.
Also adjust code to allow no entries in the "origin priorities" to help
test what happens when auto time isn't supported (see the associated
change in packages/apps/Settings/

Tested with:

adb shell cmd dumpsys time_detector
<Inspect the ConfigurationInternal>
<View SettingsUI Date & Time screen>

adb shell cmd device_config system_time time_detector_origin_priorities_override _[]_

adb shell cmd dumpsys time_detector
<Inspect the ConfigurationInternal>
<View SettingsUI Date & Time screen>

Bug: 172891783
Test: See above
Change-Id: I1e2573384055b2527f39b0134ace5f08019a3a40