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To speed up the boot process, Android doesn't visit every directory
in /sys. Instead, only those directories which match a regular
expression in /file_contexts are visited. Other directories are
skipped. This results in 2-3 second boot time reduction.

The initial version of this optimization was implemented in
change 0e7340fb. However, because
PCRE wasn't available, it was recognized that false positives and
false negatives might occur.

Now that PCRE is available, start using it. It will avoid the
false positive / negatives problem.

Bug: 17682157

(cherry picked from commit d0b768ab)

(cherry picked from commit d514c5af)

Change-Id: I78a0453236d2c2ebf7a5dcd44f896c06a2b423c5