Add support for serialized property contexts

This adds support for reading a serialized
/dev/__properties__/property_info file, which contains a
serialized trie that maps property names to the SELinux context to
which they belong.

Performance wise on walleye, this change reduces the start up cost in
libc from ~3000us to ~430us.  On a benchmark that calls
__system_property_find() for each property set on the system, it
reduces the time per iteration from ~650us to ~292us.

Bug: 36001741
Test: Boot bullhead, walleye, run unit tests
Test: Benchmark initialization and lookup performance

Change-Id: I0887a3a7da88eb51b6d1bd494fa5bce593423599