Refactor generate-image-prop-dictionary.

It's really error prone to add new partitions or image flags given the
amount of code duplication here. Since most images have (or should have)
roughly the same flags, this factors the work out into a helper
function.

Bug: N/A
Test: m, m otapackage
Change-Id: If1a22b9d7b5cf028ba52608322c4383792dae6b9