Factor out a system_properties interface for interacting with getprop/setprop.

Also, teach it to cache read-only properties so that we don't hit the device so
often. This caching is a prereq for speed index support in Telemetry which would
like to query these often.

BUG=323813

Review URL: https://codereview.chromium.org/99713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238601 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238601