Maintain existing train information

When sendBinaryPushStateChangedAtom is called with empty train
name/version/experiment ids, populate the empty fields from the
trainInfo previously stored on disk if possible.

If no train info is on disk (or read fails), the empty fields are passed
through.

Test: gts in ag/6731864
Test: testdrive 10051 with partially/completely missing train info
Bug: 128640419

Change-Id: Iea3c2db5c11d07f0b42b41aec7b2d7913a99ca03