Extend timeout for receiving broadcasts

The test is currently flaky as broadcasts can take a long time
to be delivered, often more than 500ms.
Extend the timeout for broadcasts to 30s.

Historically the test would wait for broadcasts for at most 500ms,
but not verify that they were received before proceeding. This was
fixed recently by adding an assertion, but the assertion needs to
give enough time for the broadcast to be received.

Bug: 173331190
Change-Id: I0760f3418c635aa4d68de5aefdb7120a9d3e930b
Test: atest FrameworksNetTests