Add logic to create screenshot file

* Creating screenshot file for all types of bugreport
* The logic to populate the screenshot file is in dumpstate (few
bugreport types that do not need screenshots will leave the file
untouched/empty)
* Delete the empty screenshot file and set the file to null so that
empty file does not get shared
* This TODO was previously left as binder did not allow to send null
screenshotfd, which was recently fixed. While fixing and testing the
optional screenshot fd CL with these changes, left the value "null" by
default. Fixed now.

* Not adding any logic here on the basis of the type of bugreport as
all that is already in dumpstate

* Making the names of bugreport (.zip file) and bugreport screenshot
(.png file) consistent. In the old flow also both have the same names.

Bug: 123617758
Test: * Build and flash to the device
      * Turn on use bugreport API feature flag from Settings
      * Take interactive bugreport
      Expectation: On clicking the notification to "Share the bugreport"
      only 1 file (the bugreport zip file) gets shared. Also, there is
      no screenshot file in /bugreports/ in the device
      * Take full bugreport
      Expectation: On clicking the notification to "Share the bugreport"
      2 files (bugreport zip and screenshot png file) get shared. Also
      we can see the screenshot file in /bugreports/ in the device

Change-Id: I39cb6913033b6e3c61bd0c6655a76524c12d2428