Merge all install stages in a single file

In Kotlin, we can use a sealed class to define a super class and keep all its
subclasses in the same file. As such, merge all files in the
installstagedata/ inside InstallStages.kt. Accordingly, import the right
classes where required.

Bug: 182205982
Test: builds successfully
Change-Id: I706f1edc145ab3c9efcb6fb3aeaac09c80420ccc