ANDROID: ashmem: Add support for disabling unpinning feature

The final version of ashmem will not support unpinning buffers.
Therefore, to be able to have the ashmem driver behave as close as
possible to its final configuration for testing, add a device node
that can be used to disable unpinning.

This node will make it so that the ashmem shrinker stops running,
and that all unpinning requests are ignored.

Bug: 111903542
Change-Id: I99ae9b1a4e56ee8a5224d647a6f2f9eeeb86ef02
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>