JDK 9 RFR of JDK-8033712: Fix more serial lint warnings in sun.awt

[Joe Darcy]

Hello,

Please review my patch to fix the serial warnings in platform-specific 
code in the sun.awt package:

    JDK-8033712: Fix more serial lint warnings in sun.awt
    http://cr.openjdk.java.net/~darcy/8033712.0/

Thanks,

-Joe