soong config: add value_variable substitution

There are some cases that aren't handled with the existing variable
types for booleans or known lists of strings. Similarly to our
product_variables that uses %s / %d for things like
PLATFORM_SDK_VERSION, allow vendors to define their own config variables
to be substituted into properties.

For example, some of the makefiles that I've attempted to convert had
the option to pass in version numbers from the board, or the default
display size:

  -DDISPLAY_VERSION=550
  -DDISP_H=1080

These examples happen to be integers, but since our configuration
language (make) doesn't support numbers, %s works just as well.

This change will allow the above to be represented using:

  soong_config_module_type {
      name: "acme_cc_defaults",
      module_type: "cc_defaults",
      config_namespace: "acme",
      value_variables: [
          "DISPLAY_VERSION",
	  "DISP_H",
      ],
      properties: ["cflags"],
  }

  acme_cc_defaults {
      name: "my_defaults",
      soong_config_variables: {
          DISPLAY_VERSION: {
	      cflags: ["-DDISPLAY_VERSION=%s"],
	  },
	  DISP_H: {
	      cflags: ["-DDISP_H=%s"],
	  }
      },
  }

Test: built-in tests
Change-Id: I18f35746b5cc39c304a136980249e886d38c6df6