Add tests for optional screenshot Fd in bugreport API

Not all bugreports need screenshots, and hence it should not be
mandatory to pass screenshotfd in parameters to startBugreport (call to
bugreport API).
Ensuring the above by passing null screenshotFd.

Bug: 123578274
Test: atest BugreportManagerTest
Change-Id: I79f4a71cc79e655a64c4cf9ac04fef0814db8ff1
1 file changed