Prevent crash in CoreSettingsObserver

When any setting changes, CoreSettingsObserver enumerates all
virtual devices and fetches the setting value for every virtual
device id (by creating an explicit device context and calling
Settings#getString with that context). However, by the time it
calls Context#createDeviceContext with a device id, it is possible
(though rare) that the virtual device is gone. This CL handles that
exception and skips the fetching of settings for any invalid device
id.

Test: atest CoreSettingsObserverTest
Fixes: 425014464
Flag: android.companion.virtualdevice.flags.device_aware_settings_override
Change-Id: I0a39d4208fef0d4cfe46a2c487ffc27a14ccdd57