Move away from device.get_props.

Properties can include \n so get_props isn't sound, and it's not obvious how
to fix it (without adding something like cat's -v to getprop, which wouldn't
help the intended use case of working with arbitrary old devices).

Bug: https://issuetracker.google.com/37093233
Test: gdbclient -r /system/bin/date
Change-Id: I96042fd44f1de17974968954fab6c916a9d13b61