Add option to use default Java import optimizer

[ohle]

With v1.16.0, the Intellij plugin started automatically reformatting imports with no option to disable that. Before v1.16.0, it was possible to use custom import rules via Intellij settings, but these are now overridden by google-java-format.

See for example issue #986. This means that for code bases that were using google-java-format <1.16.0 and custom import rules, it is impossible to update google-java-format.

This pull request adds a checkbox to the Intellij plugin's settings (default on) that, when off, will fall back to Intellij's default import optimizer, enabling such projects to use current versions of google-java-format with their custom import rules.

[google-cla]

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