Improve for support for Android < 8.0 (Fix NoClassDefFoundError)

[vonox7]

java.nio.file is only available starting at android API level 26 (= Android 8.0), see https://developer.android.com/reference/java/nio/file/package-summary.
According to Google, 17,3% of all android phones currently (December 2021) in use are below Android version 8.0.
Note that java.nio.channels and java.nio.charsets are available since Android API level 1 (= Android 1.0).

Moving the usages of java.io.file into an own class omits a java.lang.NoClassDefFoundError during runtime. Previously this error gets thrown when MicsUtil was first loaded from classpath (e.g. when using MiscUtil.valueNullOrEmpty() in Email.java). Now with this PR all java.nio.file imports happen only in one file, which is not loaded during runtime when not using the CLI (which you most probably don't do on Android).

[bbottema]

These methods are used from a lot of places actually. For example emailBuilder.withPlainText(File file). I suppose with some careful API usage you could avoid these methods, but it's not as straightforward as "not using the CLI".

Other than that I think it's generally a good idea to separate these methods out to a FileUtil.

[vonox7]

Yes exactly :)
But as long as you don't work with files it would be then useful not to rely on File classpath-dependencies to avoid crashing on Android for things that are not used :)

[bbottema]

Released in 6.7.0