Correctly handle change notifications during schema migrations.

Our documentation promises that only SchemaChangeInfo will be dispatched
during schema migrations, but the implementation was that individual
DocumentChangeInfos were being dispatched for every migrated document,
contrary to our documentation.

Bug: 193494000
Test: GlobalSearchSessionCtsTest#testRegisterObserver_schemaMigration
Change-Id: I73e60876c12e6240f9b7782f9c58fc9888aaef3a
12 files changed