Use dev/null instead of tmp file

Use dev/null fd (instead of using fd from temporary file) when
screenshot fd is passed as null, as file deletion does not need
to be handled for dev/null.

Test: Take interactive bugreport (which does not need screenshot)
manually using shell (which calls bugreport API)
Bug: 128981582
Merged-In: I1719899e6cf3bffe1a96849691c051ff4f3a05ec
Change-Id: I1719899e6cf3bffe1a96849691c051ff4f3a05ec