Change FileDescriptor to PFD, per API guidelines.

handleShellCommand() would be a system API, so we should use PFD instead,
per the API guidelines.

Bug: 142751512
Test: "adb shell cmd jobschduler help" -> show help
Test: "$ cmd jobscheduler run abc 123" -> shows "Package not found: abc / user 0"
Test: "adb shell cmd activity help" -> show help
Change-Id: I64b1e4c540db836cc4a46fed24a038b7077ecb63