Log source/target build version to last_install

Parse the build.version.incremental from the metadata of the update
package; and log it to last_install.
Example:
In metadata we read:
post-build-incremental=2951741
pre-build-incremental=2943039

In last install we log:
source_build: 2943039
target_build: 2951741

Bug: 28658632
Change-Id: I0a9cc2d01644846e18bda31f4193ff40e8924486