System.getProperties shouldn't return different map instances

[mlopatkin]

The result of System.getProperties() can be synchronized on. To make it work properly in build scripts, the same instance must be returned each time the method is called. However, the current instrumentation implementation returns different instances each time, making proper synchronization impossible.

Gradle runtime may also access system properties. Such calls aren't instrumented and use the underlying system properties instance. Ideally, they should share the monitor.

[mlopatkin]

The problem here is that we have consumer variable captured inside the returned instance, which corresponds to the callsite.