InputVerifier: add panic message for source conversion

We've seen a crash due to this failing, without many details on why, so
add an explicit check for the conversion with a more descriptive message
for next time.

Bug: 397706159
Test: modify InputVerifier.cpp to pass an invalid source, enable
      verification, and check that any input event causes a panic with
      the appropriate message:
      $ adb shell 'stop && setprop log.tag.InputDispatcherVerifyEvents
                                   DEBUG && start'
Flag: EXEMPT log only update
Change-Id: I0aa07cd14c8afdcd204daa0fb4660466e6c0be42