Add app widget system theme, support Android 12 system theme

[UweTrottmann]

https://developer.android.com/about/versions/12/features/widgets https://github.com/android/user-interface-samples/tree/main/AppWidget

• Use Android 12 reconfigure button.
• Use Android 12 preview.
• Update pre-Android 12 preview image.
• Limit upcoming/recent item count to 100 to cap memory usage (might otherwise return thousands of items with infinite calendar option).

[github-actions[bot]]

Unit Test Results

  6 files  ±0    6 suites  ±0   8s :stopwatch: -1s 16 tests ±0  16 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit f491deb2. ± Comparison against base commit 5f29c7cf.

:recycle: This comment has been updated with latest results.