Move BitUtils, RingBuffer test to coretests

The utilities are part of com.android.internal.util, but were
historically tested as part of the services.net tests. They were then
mistakenly moved to packages/modules/Connectivity with the rest of the
networking code, but actually belong in coretests.

Bug: 187935317
Test: atest FrameworksCoreTests
Change-Id: Ia4179ac3e25170e46b47133be0f5d4f1b24caf3d