Fix Configuration.diff documentation.

By using ActivityInfo.Config to annotate the return type of
Configuration.diff, it not only solves the wrong document but also
makes the documentation of Configuration.diff consist with
the documentation of Configuration.updateFrom.

The final result is the following.
```
Returns a bit mask indicating which configuration values have changed.
Value is either 0 or a combination of ActivityInfo.CONFIG_MCC,
ActivityInfo.CONFIG_MNC, ActivityInfo.CONFIG_LOCALE,
ActivityInfo.CONFIG_TOUCHSCREEN, ActivityInfo.CONFIG_KEYBOARD,
ActivityInfo.CONFIG_KEYBOARD_HIDDEN, ActivityInfo.CONFIG_NAVIGATION,
ActivityInfo.CONFIG_ORIENTATION, ActivityInfo.CONFIG_SCREEN_LAYOUT,
ActivityInfo.CONFIG_UI_MODE, ActivityInfo.CONFIG_SCREEN_SIZE,
ActivityInfo.CONFIG_SMALLEST_SCREEN_SIZE, ActivityInfo.CONFIG_DENSITY,
ActivityInfo.CONFIG_LAYOUT_DIRECTION, ActivityInfo.CONFIG_COLOR_MODE,
and ActivityInfo.CONFIG_FONT_SCALE
```

Test: make docs
Test: review reference/android/content/res/Configuration.html in
      out/target/common/docs/online-sdk-docs-docs.zip

Fixes: 171738297
Bug: 171716892
Change-Id: Iaa2ac25bc1269e4a9a625cce386f51984ffff923