tools: Fix art script to run from adb shell interactively

Previously it would not work for two reasons:

1) It assumed core.art image, but a regular android device only has
boot.art
2) It assumed being located in a writeable directory, and would fail
to create //android-data12345 or similar (notice the //).

Test: adb shell /system/bin/art -cp /data/local/tmp/classes.dex 'Outer\$Inner'
Change-Id: I4a5560cab4f0e3e400551f435d7d0ec9f3d505ba