Fix "adb remount" for devices without an oem partition.

On a device without an oem partition, we now have an /oem directory
anyway. This causes find_mount to fail, and that was returning nullptr
from a std::string-returning function. Boom!

Also clean up the bits of code I had to trace through between "adb remount"
on the host to the crash on the device as I debugged this.

The only other meaningful change is the error checking in
adb_connect_command --- adb_connect can also return -2.

Bug: http://b/20916855
Change-Id: I4c3b7858e13f3a3a8bbc7d30b3c0ee470bead587