Give uncrypt access to /proc/cmdline.

Removes uncrypt access to the rest of /proc.
Fixes this denial, which can be triggered from recovery_component_test:
[  142.540819] type=1400 audit(1506983074.139:23): avc: denied { read }
for pid=5767 comm=uncrypt name=cmdline dev=proc ino=4026532114
scontext=u:r:uncrypt:s0 tcontext=u:object_r:proc_cmdline:s0 tclass=file
permissive=0

Bug: 66497047
Test: recovery_component_test --gtest_filter=UncryptTest.*
no more denials to /proc/cmdline
Change-Id: If1a7630779d667d52a0cc44114ef6177982de21c