Stage any flag on DeviceConfig writes.

Currently, flags are staged to be on reboot by being written to the
staging namespace. The server-side explicitly writes to the staging
namespace. That means flags can still be toggled immediately locally,
and when the server-side doesn't have staging enabled. A race condition
on the first wifi connection means that sometimes flags can get
immediately toggled. This CL makes all writes stage, even local ones.

Bug: 326598713
Test: atest SettingsStateTest
Change-Id: I9a551843ef710bf074b40cbe581caf075cfafc07