Add TestableDeviceConfig

Allow for better unittesting when using DeviceConfig. This allows unit
test to run without having to interact with the real values on the
device.

Ported AppCompactorTest as an example.
Forked DeviceConfigTest into TestableDeviceConfigTest.

Bug: 123035013

Test: atest AppCompactorTest TestableDeviceConfigTest
Test: atest android.provider.DeviceConfigTest

Change-Id: Ibc54cffd8e009c55ee95b87995843c9938c67292