Don't reformat 'javadoc' comments that appear before package/imports, to avoid mangling license headers

The use case is license headers added by, for instance, maven [license](http://www.mojohaus.org/license-maven-plugin/) [plugins](https://github.com/mycila/license-maven-plugin). These headers are not by default formatted in the way that google-java-format requires, and since neither (for good reasons - neither code styles nor licenses should be changed lightly) are very configurable, conflicts are hard to avoid. (The mycila license plugin allows configuration, and with a bit of effort, you can make it create license headers that don't get reformatted - I haven't succeeded to do that with the Codehaus plugin).

Creating this issue as a different one than #139, which is about javadoc comments.

Start by reproducing the issue with a license header formatted differently from google-java-format before the package or imports. Trace how the formatter handles that leading comment, and consider the issue resolved when such headers remain unchanged while the rest of the Java source is formatted.

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