Show bugreport progress.

adb calls bugreportz to generate a bugreport; initially, bugreportz
would only report the final status of the operation (OK or FAIL), but
now it sends intermediate PROGRESS lines reporting its progress (in the
form of current/max).

Similarly, the initial implementation of 'adb bugreport <zip_file>'
would print an initial 'please wait' message and wait for the full
stdout before parsing the result, but now it uses a new callback class
to handle the stdout as it is generated by bugreportz.

BUG: 28609499

Change-Id: I6644fc39a686279e1635f946a47f3847b547d1c1