Clean up code of starting bug report

- Let screenshot fd be null if Shell doesn't want to take default screenshot when bug report is triggered.

BUG:149525300
Test: Flash and test interactive/full bugreports generated using Shell, and Shell flow does not break during tests
Change-Id: I50908af63d74ed66156354ec0b5a237f7131061a
Merged-In: I50908af63d74ed66156354ec0b5a237f7131061a