Add EmailHelper.getLocalPartFromEmailAddress()

package com.fsck.k9.helper

object EmailHelper {

    @JvmStatic

    fun getLocalPartFromEmailAddress(email: String): String? {

        val index = email.lastIndexOf('@')

        return if (index == -1 || index == email.lastIndex) null else email.substring(0, index)

    }

    @JvmStatic

    fun getDomainFromEmailAddress(email: String): String? {

        val index = email.lastIndexOf('@')

        assertNull(result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withRegularEmail_shouldReturnLocalPart() {

        String result = EmailHelper.getLocalPartFromEmailAddress("user@domain.com");

        assertEquals("user", result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withAtInLocalPart_shouldReturnLocalPart() {

        String result = EmailHelper.getLocalPartFromEmailAddress("\"user@work\"@domain");

        assertEquals("\"user@work\"", result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withInvalidEmail_shouldReturnNull() {

        String result = EmailHelper.getLocalPartFromEmailAddress("user");

        assertNull(result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withEmptyDomain_shouldReturnNull() {

        String result = EmailHelper.getLocalPartFromEmailAddress("user@");

        assertNull(result);

    }

}