Implement new "maps" property in Root

Add "maps" as an allowed root value to template data JSON.

Maps allow applications to map equivalencies between their
application-specific parameters and valid template parameters.

Each Map has arbitrary keys (from the consuming application) that
are mapped to one or more template parameters.

* Added root.maps to TemplateDataBlob
  * Validates that values are strings or arrays
  * Validates that values are valid params
* Added templatedata-invalid-param to i18n/en.json
  * Richer error message for when invalid value is a param
* Fixed existing tests to work with new "maps" property
* Added two new tests for maps
  * Test for invalid value (!string or !array)
  * Test for invalid parameter
* Added specification for root.maps property and Map object

Change-Id: I3bf5e002ad6c1632e02c4c2e393b244c51f77177
Merge "Implement new "maps" property in Root"