Switch android.webkit.MimeTypeMap over to libcore.net.MimeUtils' table.

(Said table is the very same data that used to be, but pushed further down
the dependencies so that it can be shared with java.net.URLConnection, so
the two classes' equivalent methods return the same results.)

Bug: 2422917
Bug: http://code.google.com/p/android/issues/detail?id=10100
Change-Id: Ic4e80c2fede22671e825d569f349e4329dcf01ea