Allow inputflinger to call system_server.

Resolves denials such as:
avc:  denied  { read } for  pid=752 comm="ActivityManager" name="stat" dev="proc" ino=1878 scontext=u:r:system_server:s0 tcontext=u:r:inputflinger:s0 tclass=file
avc:  denied  { open } for  pid=752 comm="ActivityManager" name="stat" dev="proc" ino=1878 scontext=u:r:system_server:s0 tcontext=u:r:inputflinger:s0 tclass=file
avc:  denied  { search } for  pid=752 comm="ActivityManager" name="214" dev="proc" ino=1568 scontext=u:r:system_server:s0 tcontext=u:r:inputflinger:s0 tclass=dir
avc:  denied  { read } for  pid=752 comm="ActivityManager" name="stat" dev="proc" ino=1878 scontext=u:r:system_server:s0 tcontext=u:r:inputflinger:s0 tclass=file
avc:  denied  { call } for  pid=187 comm="Binder_2" scontext=u:r:inputflinger:s0 tcontext=u:r:system_server:s0 tclass=binder

Change-Id: I099d7dacf7116efa73163245597c3de629d358c1
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>