Rename Evaluate() to Get() and add GetDefault()

Part of the design of property structs is that they were easy to access.
In keeping with that spirit, use a shorter and easier to spell name
for the getter, and add GetDefault() so that you don't need to pass
the result of Get() to one of the
proptools.StringDefault/BoolDefault/etc functions.

Bug: 323382414
Test: m nothing --no-skip-soong-tests
Change-Id: I0b8df151de59f2cd9463425c6666cbfda6bf47f2