Add build.version.incremental to metadata

Add the build property "build.version.incremental" of the source (if
present) and target files to the metadata of the ota update package.

Example of metadata:
....
post-build-incremental=2951741
post-timestamp=1465345123
pre-build-incremental=2943039
pre-device=bullhead
...

Bug: 28658632
Change-Id: I889e8ccf39633b1b35590751001a42d1b05d5514