docs: fix code snippets in SQL Databases page

In the "Saving Data in SQL Databases" page, this commit makes the
following updates:
-the create snippet now includes the subtitle column
-the insert and query snippets no longer references undefined variables
-the delete snippet now references the table with "FeedEntry.TABLE_NAME"

Bug: 25856493
Change-Id: I4fdf8e271a1141776c2427c0b3fef8ec403dc423