Don't fire 'changed' event if printer list is not updated

The printer list is tried to be updated when the network is changed
but the 'changed' event was fired even if the printer list is not
changed at all.

This CL suppresses events if the list is not changed.

Bug: 132840441
Test: run 'cts-tradefed run commandAndExit cts-dev --module
    CtsPrintTestCases --test android.print.cts.
    PrintDocumentAdapterContractTest#unexpectedLayoutCancel' 30 times
Change-Id: I9236e56ea9e19e37be29d6a1d0015ee0b2676460