experimental on-device log-saving script

Add script that when copied to device and run from 'adb shell' captures
all app and OS logs to file. Next: add code to XWApp.java to copy it
from assets/ and run it on launch.

xwords4/android/app/src/debug/assets/startlog.sh

@@ -0,0 +1,11 @@
+#!exec sh
+
+set -e -u
+
+APP=${1:-APP}
+
+mkdir -p /sdcard/${APP}_logs
+cd /sdcard/${APP}_logs
+
+FILENAME="$(date +%d-%m-%Y_%H%M%S).log"
+logcat -v time >> $FILENAME