Move RingBuffer to SystemUICommon.

This change adds a new module SystemUICommon and moves RingBuffer to
it. This is to prepare introducing a new logging library which would
depend on it.

Bug: 276475093
Test: RingBuffer does not have existing tests, so just verified
LogBuffer works correctly on device

Change-Id: Ib71fcf8526021be1ac3fe279210c54cb7d4cd0e6