image_generator: Fix the warnings on import order.

[platform/bootable/recovery] tools/image_generator/ImageGenerator.java:36: Wrong order for java.util.StringTokenizer import. Use Ctrl+Shift+O (Eclipse) or Ctrl+Alt+O (Intellij) to sort imports. https://source.android.com/setup/code-style#order-import-statementsERRORS:
[platform/bootable/recovery] tools/image_generator/ImageGenerator.java:43: Wrong order for org.apache.commons.cli.CommandLine import. Use Ctrl+Shift+O (Eclipse) or Ctrl+Alt+O (Intellij) to sort imports. https://source.android.com/setup/code-style#order-import-statementsERRORS:
[platform/bootable/recovery] tools/image_generator/ImageGenerator.java:50: Extra separation in import group before 'org.w3c.dom.Document'

Test: `mmma -j bootable/recovery`
Test: `repo upload` no longer gives warnings.
Change-Id: If24c6b7ca33b9223b3e326a48885c24c35b5fa68