Prevent MockSpdyPeer interfering with Android tests

On Android we run the OkHttp tests as an instrumented test case.
The default UncaughtExceptionHandler on Android will quit the
app, causing the current test to fail.

MockSpdyPeer creates a thread that fails for various reasons
that are not related to / required for the actual test. Often the
exception is thrown during a subsequent test.

This change logs the exception but prevents it from
propagating out of the top of the call stack. This decreases
overall flakiness.