Refactor InstallationAsyncTask.Progress

Instead of passing a number around as the number of installed
partitions, track the number in a member field in InstallationAsyncTask.
This makes installing partitions after installImages() had returned
possible.

Bug: 165925766
Test: TH
Change-Id: Iec45c9fa1a39f0bcc4eeaa00e44d3394d55087d3