Add a wrapper class PartitionBuildProp

The build prop for a partition used to be a simple key:value
dictionary. But we need more fields to hold the alternative build
props overriden by the 'import' statement. Therefore, add a new
class as a wrapper for these props first.

Bug: 152167826
Test: unittests pass
Change-Id: I2fe7e93a2f4de8e55f5f8051b000b96b5efdc85a