Additional cleanup of start_device_log.

Addresses nnk's post commit review comments on
https://android-review.googlesource.com/#/c/139381/

Remove unneeded code for creating /data/adb.

Add an O_CLOEXEC.

Move the closing of stdin out to main().

Append the pid of the current process to the log file to avoid
clobbering the log if the process crashes and restarts within the same
second.

Change-Id: Ide0be86b4b33256486634c29ba02efaf10cf913d