Do not create Clipboard entries for invalid keys.

This fixed memory leak caused by asynchronous calls to
getClipboardLocked scheduled on HandlerThread which
were causing that previously clean-ed up entries corresponding
to released VirtualDevice instances were re-created and never
cleared.

Similar memory leak can happen when getClipboardLocked is called
for stopped user after clipboard were already cleared for the user.

Bug: 290350147
Test: atest StreamedAppClipboardTest --iterations 100 & heapdump check
Change-Id: Ie7e972f40411685199d9fea4cc6f62928e7580b4