sysprop_library: Rename system scope to public

In the first design, public sysprops have been accessible from Java
modules linking against SDK. But SDK modules shouldn't do, because
sysprop_library isn't for the apps. This renames system to public, so
that only public(System till now) and internal scopes remain from now.

Bug: 131637873
Test: m && sysprop_test
Change-Id: I548007d4a6018922f98d3d13915cee1d66070086