Wait for broadcast idle instead of sleep

In most cases, this change will make this test run faster,
as the device isn't processing a lot of broadcasts so the
broadcast queue becomes idle much earlier than 5 seconds
(tests on a Pixel 4A running AOSP indicate ~1 second).

On low-end devices, this will give the device a better
chance to actually pass the test in spite of high load.

The drawback is that this will wait for the broadcast
*queue* to be idle, so for all broadcasts to have been
delivered to all apps, not just the one this test is
interested in. To wit, there will be cases where this
will increase the length of the test. But this should
be a minority of cases.

Test: atest ConnectivityDiagnosticsManagerTest
Bug: 257412943
Change-Id: I84a8929635ee3a587ef2712a59323abe68fda31b