android: binder: support multiple /dev instances.

Add a new module parameter 'devices', that can be
used to specify the names of the binder device
nodes we want to populate in /dev.

Each device node has its own context manager, and
is therefore logically separated from all the other
device nodes.

The config option CONFIG_ANDROID_BINDER_DEVICES can
be used to set the default value of the parameter.

This approach was favored over using IPC namespaces,
mostly because we require a single process to be a
part of multiple binder contexts, which seemed harder
to achieve with namespaces.

Change-Id: I3df72b2a19b5ad5a0360e6322482db7b00a12b24
Signed-off-by: Martijn Coenen <maco@google.com>