Merge all uninstall 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
uninstallstagedata/ inside UninstallStages.kt. Accordingly, import the right
classes where required.

Bug: 182205982
Test: builds successfully
Change-Id: I9a0bcdb34e2f00cd6a4da80469293adf43b740e7