Image state restoration for LongScreenshotActivity

- When long screenshot is acquired, save image to a temporary file.
- When restoring state, read in that bitmap.
- Moved bitmap export code from ScrollCaptureController to
  LongScreenshotActivity because it needs to be able to export any
  drawable regardless of source.
- Currently deleting the temporary file onDestroy.

Bug: 181087631
Test: Enable "don't keep activities" debug setting. Take long
screenshot, go to overview and return. Rotate UI. Verify that image
restores along with crop bounds and export still works properly.

Change-Id: I892ad78025c818c8c65d16f2844daa955cc0b005