Improve testing of CONNECTIVITY_ACTION broadcasts.

We currently test CONNECTIVITY_ACTION broadcasts by directly
registering BroadcastReceivers with BroadcastInterceptingContext,
and making the receivers unregister themselves when all the
broadcasts they expect have been received.

This works for current test cases, but does not work if anything
registers another receiver for CONNECTIVITY_ACTION. In that case,
when we unregister the receiver in the receiver's onReceive
method, BroadcastInterceptingContext will throw a
ConcurrentModificationException because the list of receivers is
being modified during iteration.

Fix this by adding an ExpectedBroadcast class that stores the
receiver and unregisters the receiver only when the test checks
that the broadcast was received, which happens after the receiver
runs. This is easier to use and also guarantees that the receiver
is unregistered even if the test is expecting that the broadcast
is never fired. Accordingly, remove mRegisteredReceivers and the
code that uses it; it's no longer necessary now that
ExpectedBroadcast always unregisters its receivers.

Also add a convenience expectConnectivityAction method to expect
a CONNECTIVITY_ACTION broadcast with specific contents. This
makes the test easier to read and more detailed. Convert some
existing tests to this method.

While I'm at it, fix a test that was using "mCellNetworkAgent" to
represent a wifi network.

R backport notes: added import for NetworkInfo.DetailedState.
That was added in aosp/1527378, which is impractical to backport.

Bug: 172870110
Test: test-only change
Change-Id: Ibada8b4215625e1016d9fd170526206920af76f5
Merged-In: Ibada8b4215625e1016d9fd170526206920af76f5
(cherry picked from commit d39cfb8d)