Fix back stack didn't perserved when Files Activity launched by Setting storage then back case.

Due to FilesActivity set "android:documentLaunchMode="intoExisting", it will create
new task when launched from Storage Setting page.

When select FilesActivity task from recents & press back key, it will not back
to settings page.

Using startActivityForResult to set task as source task can fix task separated case.

Change-Id: I26362928261a1a9c0e485a486be493423d1fcf61
Fixes: 33117269
Test: Manual
Test: atest StorageItemPreferenceControllerTest