Remove XML write functions from UsageStats.

UsageStats has moved away from XML and is now using protos. This removes
dead XML write functions which should ideally never be triggered. A
warning will be logged when reading XML data and a WTF when some
codepath attempts to write data in XML.

Certain tests are now marked as ignored since the XML write functions are
removed. It was decided to not move those functions into the test context
since the read/write code is so intertwined within the database logic.

Bug: 143187610
Test: atest UsageStatsDatabaseTest
Change-Id: If622b646e0edfe389e49aec9b51accdb9fbf1f45