Add std::string system property functions.

Makes it easier to write correct code in a world where the maximum
property key/value lengths change.

Bug: http://b/23102347
Test: libbase_test64

(cherry picked from commit 1e88c8c0ac31a4b07da4228ed9ac41e50ae56886)

Change-Id: I6f572605220319cfe87facfd5361c4ad9895a33c
4 files changed