os.linux: Move Attributes class to internal package

os.linux: Move Attributes class to internal package

The os.linux.kernel analysis should eventually encapsulate all
its state system accesses, which means clients should not call
the Attributes class directly. Move Attributes class to the
analysis's internal package so it gets removed from the API.

We can make some known users x-friends for now, but eventually
they should be moved to using the real API.

Change-Id: I0b78c8ff88b039311d89d6313d40c2e9381c29af
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/70424