Upgrade PostBuildScript and Update jjb to 2.0.3

The PostBuildScript plugin 2.x is a major update and the XML
configuration is not back compatible. That also requires jjb 2.0.3 in
order to generate the new XML config.

The YAML configuration is not too difficult, the change is best viewed
ignoring whitespaces (git show -w).

script-only-if-succeeded and script-only-if-failure were somehow
confusing. The new build-on is straightforward, you just explicitly list
all the statuses you want the steps to build on. Pick from:

SUCCESS, UNSTABLE, FAILURE, ABORTED, NOT_BUILD

I have split some postbuildscript since we want some steps to mark the
build unstable on failure, but some other steps we can ignore their
result entirely (ex: garbage collecting the coverage output).

Bug: T188398
Change-Id: I4540f45d1761bb767f1604d722427d94042465aa