Fix invalid specification for adb_keys.

A prior change added an entry for adb_keys without any security context,
yielding warnings like the following during build:
out/target/product/manta/root/file_contexts:  line 7 is missing fields, skipping

This adds the missing security context field.

Change-Id: If48731c8aa7d22a3f547d0854f288ff68f9006da
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>