Limit upload concurrency (number of goroutines) in camput for Android

Sidestep the problem by uploading files first, and buffering
directories to upload them at the end.
Limit upload concurrency to 3+2 (normal+directories) goroutines
on Android, and -1+10 otherwise.

Change-Id: Idac4c94b7d7ec4fc6c6a77fed111e0ece0843545