Do not reinitialize system properties to fake their values

Reinitializing system properties can result in crashes later in the
program, and is generally not recommended or even supported.  Instead,
this change uses a very rudamentary approach to mocking out the system
property functions.

Bug: 62197783
Test: unit tests
Change-Id: I99c57e1d87490926e4839ee154c862ee8a199e26