Split properties into their own class to make testing better

Reinitializing system properties can result in crashes later in the
program, and is generally not recommended or even supported.  This
change moves the actual logic for system properties into a class that
can be tested in isolation, without reinitializing the actual system
property area used in libc.

Bug: 62197783
Test: boot devices, ensure properties work
Test: system property unit tests and benchmarks
Change-Id: I9ae6e1b56c62f51a4d3fdb5b62b8926cef545649