Fix getSettingsSliceUri API review issues

    1. Permissions for wallpaper settings Slice.
    2. Return value must be marked either @NonNull or @Nullable.

Solution:
    1. Add more explanation for settingsSliceUri API usage in javadoc.
    2. Follow the public API annotation rule.

Bug: 119118548
Bug: 126699851
Test: atest tests/Internal/src/android/app/WallpaperInfoTest.java

Change-Id: I5db88ce35ba11b018abe42f2192afed44a19fea5