Avoid flakiness from DeviceConfig.

Mock DeviceConfig to avoid test flakiness and failures due to relying on
the real service.

Bug: 255670453
Test: atest FrameworksMockingServicesTests:JobConcurrencyManagerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f270e394f3d9e67330a2c8a6ce17bc5dfaa62921)
Merged-In: I3a7f23b6e9c4277c9d521e7a33b29a0bbfd02331
Change-Id: I3a7f23b6e9c4277c9d521e7a33b29a0bbfd02331