Make yield-to-bad-wifi behavior backward compatible with R

Yielding cell wins to exiting wifi (whether good or bad).
It loses to bad wifi that's not exiting.

In R, yielding to bad wifi only affects wifis that are
unvalidated, but a wifi that is exiting should still be
dropped in favor of a cell that yields to bad wifi.

I had misunderstood the policy and implemented it wrong.
Now it's implemented right, and has careful tests.

Test: new tests for this
Bug: 186458024
Change-Id: Ib8637100d491e72a2edb837584ce55b7dda58524