DropBoxManager: Allow adding a file using an fd

This patch adds an overload to DropBoxManager::addFile() which accepts
an already-opened file as a file descriptor. This avoids the need for
clients to create a filesystem-visible file when uploading data to
DropBox.

Test: Tested with perfetto using https://android-review.googlesource.com/c/platform/external/perfetto/+/587674
Change-Id: I076bfd3180fb9b4baff7e1bae2e611419061b2a7