Label adb keys file and allow access to it.

The /adb_keys entry will only take effect if a restorecon is
applied by init.rc on a kernel that includes the rootfs labeling
support, but does no harm otherwise.

The /data/misc/adb labeling ensures correct labeling of the adb_keys
file created if the device has ro.adb.secure=1 set.

Allow adbd to read the file.

Change-Id: I97b3d86a69681330bba549491a2fb39df6cf20ef
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>