Thread safety issue in PresentationBuildTasks

[weltkante]

During the bugfix for #350 and #378 in PR #570 the static initialization of KnownAssemblyInfoRecord on BamlMapTable was moved into the constructor while keeping the initialized field static. This can lead to potential race conditions when the BamlMapTable is instantiated multiple times on separate threads.

Considering the post-merge comments in the PR its probably safe to make the field an instance field instead of a static field.