Parser for signed configuration.

Signed configuration is stored an JSON inside APK meta-data. The format
of the JSON is as follows:

{
  "version": n,
  "config": [
    {
      "minSdk": n,
      "maxSdk": n,
      "values": [
        {
          "key": "global settings key",
          "value": "value for key"
        },
	...
      ],
      ...
    }
  ]
}

Test: atest SignedConfigTest
Bug: 110509075
Change-Id: I4d345b1f03e5c5f5af37de093eb6e5cb2429c42c