Reland "Add a wrapper class PartitionBuildProp""

This reverts commit 60225452.

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
Change-Id: I5c952cd2a976ba1a09ddc66d56c2b8b55a61986b
Test: unittests pass