Release Mantis resources when a new file is opened

This is to handle the case where Mantis is running and the user opens another
file (e.g. a PDF) in the same window.

Also rename UntrustedProcessorTest to MantisUntrustedServiceTest which
is a leftover from a refactor https://crrev.com/c/6034010.

Test: When Mantis is running, open another file and see memory usage is lower
Bug: 368621998
Change-Id: I070b10773bd4b91025ca3a557eaa58acf860b529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6170020
Reviewed-by: Yi Chou <yich@google.com>
Reviewed-by: Patti Lor <patricialor@chromium.org>
Commit-Queue: Fairuzi Teguh <fairuzi@google.com>
Cr-Commit-Position: refs/heads/main@{#1408497}