Remove sun.misc.ManagedLocalsThread from java.prefs

JDK-8056152 added a new constructor to java.lang.Thread to constructing Threads that do not
inherit inheritable thread-local initial values. All usages of sun.misc.ManagedLocalsThread in
the base module were also replaced with this new constructor. Given there is now a supported API
for creating such threads, other areas of the JDK should be updated to use it, where applicable. If all
usages are replaced, then sun.misc.ManagedLocalsThread can be removed.

/jdk/src/java.prefs$ find . -name "*.java" -exec grep ManagedLocalsThread {} \; -print
import sun.misc.ManagedLocalsThread;
    private static class EventDispatchThread extends ManagedLocalsThread {
./share/classes/java/util/prefs/AbstractPreferences.java
import sun.misc.ManagedLocalsThread;
            Thread flushThread = new ManagedLocalsThread() {
./macosx/classes/java/util/prefs/MacOSXPreferencesFile.java
import sun.misc.ManagedLocalsThread;
                Runtime.getRuntime().addShutdownHook(new ManagedLocalsThread() {
./unix/classes/java/util/prefs/FileSystemPreferences.java