Take screenshot right away when invoked with -P.

Traditionally, the screenshot was not taken right away because it could
skew other services like the Surface Flinger, and such approach often
resulted on irrelevant screenshots.

Now there is more user-friendly workflow, where the bugreport progress
is shown in the UI and the user can take further actions. In this
workflow, it's ok to take the screeshot right away, since the other
features might already be causing interference: if the user doesn't want
interference, the old workflow should be used instead.

Since the new workflow requires the -P (for Progress) parameter, such
parameter can also be used to determine when to take the screenshot.

Also added progress statistics at the end of the bugreport.

BUG: 8420215
Change-Id: I9dad8ea59d47848f217182ccec0f527b42e07f50