Don't pretend *parser.List is immutable

The functions in bpfix that take a *parser.List and return a
modified *parser.List are always returning the same pointer
and mutating the target of the pointer.  Remove the extra
unnecessary return value.

Also extract the getLiteralListProperty function.

Test: androidmk_test.go
Change-Id: I08d8aff955c72b7916741cda8083974a49af4d6f