Simplify and correct variable assignments

- Remove asgnMaybeAppend, it was only used to indicate
  that the asignment needs a setDefault. But really, all
  types of variable assignments need setDefault if they're
  self-referential.
- Remove local_append/local_set_default because there was
  no implementation for them written in product_config.rbc
  anyways.
- Correct productConfigVariable.emitDefined using the global
  variables instead of the product config variables.
- Emit setDefaults for all types of assignments if they're
  self referential.

Bug: 222737841
Test: go test
Change-Id: I06a0f90f16d5900049d473281e6d5ef5e93e67da