Work around snapshot list segfault

Some emulator executables [exit with a segmentation fault][1] when the `-snapshot-list` option is used.

[1]: https://code.google.com/p/android/issues/detail?id=34233

Here's a proposed workaround that uses `emulator-arm` always, since there's [evidence][2] that it's sufficient regardless of AVD type and doesn't give the segfault.

[2]: https://code.google.com/p/android/issues/detail?id=34233#c57

The changes here also extend the pattern that matches snapshot strings to match decimal numbers and gigabyte units in addition to whole megabyte values.