os.linux: Do not rely on static initialisation order for layouts

os.linux: Do not rely on static initialisation order for layouts

Expose DefaultLayout#getInstance() instead of DefaultLayout#INSTANCE

This solves a weird problem where the layout could be null.

To reproduce this problem, define and run the following test suite:
@RunWith(Suite.class)
@Suite.SuiteClasses({
	VirtualMachineAnalysisTest.class,
	LttngKernelAnalysisTest.class,
	PartialStateSystemTest.class,
	StateSystemInMemoryTest.class,
	StateSystemFullHistoryTest.class
})
public class TestSuite { }

Change-Id: I772ee0944334283d4b36118001a858322921d98a
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/69864
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Tested-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-by: Hudson CI