Changed logic when dumpstate's max progress increases.

When dumpstate starts, it estimates its maximum duration and sends it
through an extra on BUGREPORT_STARTED; as it progress, it sets a system
property with its current progress and if the progress value overflows
the estimated max, it increases the max as well.

Shell uses the max/progress to display the progress % in the
system notification, and need to handle the scenario where the max
changes. The initial implementation would recalculate the progress, with
makes it swing back and forth as dumpstate increases the max.

This CL changes the Shell logic so the progress never go back, just
forward. The drawback of this approach is that if dumpstate
underestimate the maximum, the progress might get stuck in a high
value (99%) early on, but such issue will be addressed in the long
term by tuning the estimated max value.

BUG: 26354314
Change-Id: I3a5416acaffaaa43fd28d2f1f8ec8ea12aa0d91e