Add ability for flags to be server-overridden.

This works in both debug and release builds. In debug builds,
all flags can be overridden via the same mechanisms as before,
but can also be overridden by flags defined on the server.

On release builds, only `ReleasedFlag`s can be overridden.
`UnreleasedFlag`s remain set in their default state.

With this cl, server side flags do not get their new value
until SystemUI is restarted (via a reboot or a crash).

Bug: 239863271
Test: atest SystemUITests
Change-Id: I08166cb90675dea06b2671669cecfc16ac58ab61
Merged-In: I08166cb90675dea06b2671669cecfc16ac58ab61