Fixed problem where AndroidShellHolder was trashing the ThreadHost in its deconstructor

[gaaclarke]

fixes flutter/flutter#76597

There was a typo where we were clearing out a threadhost instead of reseting the reference. I removed the method since foo->Reset() and foo.reset() are very similar, in order to avoid future bugs.

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[iskakaushik]

LGTM

[zanderso]

Is it possible to add a regression test for this? Is there already an integration test that covers this?

[gaaclarke]

@zanderso There isn't an integration test that covers it. There aren't even unit tests for that class. I've found that portion of the engine (android embedder) a bit sparse with respect to tests. I remember having this discussion a couple months ago over another PR here. It might be worth revisiting.