Coverage tool: improve support for execution environments with custom class loading

[rliesenfeld]

Certain testing tools (PowerMock, Robolectric) create copies of classes through a custom class loader.

The coverage tool currently won't instrument classes whose names have already been registered as instrumented; this check should also account for different class loaders.

Also, if classes get "reloaded" without a complete ProtectionDomain, they won't be considered for coverage. This situation should also be checked for, so the reloaded classes still get instrumented.