refactor getHeaders and removeHeaders to use a common method and an

iterator to address a Market FC:

java.util.ConcurrentModificationException
at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
at com.fsck.k9.mail.internet.MimeHeader.getHeader(MimeHeader.java:87)
at
com.fsck.k9.mail.internet.MimeHeader.getFirstHeader(MimeHeader.java:51)
at
com.fsck.k9.mail.internet.MimeMessage.getFirstHeader(MimeMessage.java:437)
at
com.fsck.k9.mail.internet.MimeMessage.getContentType(MimeMessage.java:130)
at
com.fsck.k9.mail.internet.MimeMessage.getMimeType(MimeMessage.java:159)
at
com.fsck.k9.mail.internet.MimeUtility.findFirstPartByMimeType(MimeUtility.java:971)
at com.fsck.k9.crypto.Apg.isEncrypted(Apg.java:464)
at
com.fsck.k9.activity.MessageView.updateDecryptLayout(MessageView.java:2702)
at
com.fsck.k9.activity.MessageView$Listener$6.run(MessageView.java:2466)
at android.os.Handler.handleCallback(Handler.java:587)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:144)
at android.app.ActivityThread.main(ActivityThread.java:4937)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)