Add conditions_default for soong config variables.

Each variable can specify a conditions_default for properties to be used
when the variable is not set, not set to a true value (for bools), or is
set to a value that is not present in the module (for strings).

Test: m nothing
Test: go test soong tests
Change-Id: I76ec026da2369b407f0f530f77760f530e7958fc