Generate a general_seapp_contexts file for tests.

Generate a general_seapp_contexts file with only the
device-independent entries, similar to general_sepolicy.conf.
This is for use by CTS tests to compare with the prefix of
device seapp_contexts.

Change-Id: If8d1456afff5347adff7157411c6a160484e0b39
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>