Disallow empty eventTimezone values. Do not merge

We now reject insert and update requests from apps that could result
in an Event with a null/empty eventTimezone column.

Bug 5514124 and 5497100

Cherry pick of https://android-git.corp.google.com/g/#/c/145982/

Change-Id: I5f3d1f5f797b5d1da3b24c4ca85d9afaa90449dc
1 file changed