android: binder: move global binder state into context struct.

This change moves all global binder state into
the context struct, thereby completely separating
the state and the locks between two different contexts.

The debugfs entries remain global, printing entries
from all contexts.

Change-Id: If8e3e2bece7bc6f974b66fbcf1d91d529ffa62f0
Signed-off-by: Martijn Coenen <maco@google.com>