Make Localization.relativeTime testable

Problem is global state in static variable prettyTime. But for performance reasons on Android that is preferred.
Now allow injecting prettyTime dependency by making init function public.

app/src/test/java/org/schabi/newpipe/util/LocalizationTest.kt

@@ -0,0 +1,39 @@
+package org.schabi.newpipe.util
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.ocpsoft.prettytime.PrettyTime
+import java.text.SimpleDateFormat
+import java.time.OffsetDateTime
+import java.time.ZoneOffset
+import java.util.GregorianCalendar
+import java.util.Locale
+
+class LocalizationTest {
+
+    @Test
+    fun `After initializing pretty time relativeTime() with a Calendar must work`() {
+        val reference = SimpleDateFormat("yyyy/MM/dd").parse("2021/1/1")
+        Localization.initPrettyTime(PrettyTime(reference, Locale.ENGLISH))
+
+        val actual = Localization.relativeTime(GregorianCalendar(2021, 1, 6))
+
+        assertEquals("1 month from now", actual)
+    }
+
+    @Test(expected = NullPointerException::class)
+    fun `relativeTime() must fail without initializing pretty time`() {
+        Localization.relativeTime(GregorianCalendar(2021, 1, 6))
+    }
+
+    @Test
+    fun `relativeTime() with a OffsetDateTime must work`() {
+        val reference = SimpleDateFormat("yyyy/MM/dd").parse("2021/1/1")
+        Localization.initPrettyTime(PrettyTime(reference, Locale.ENGLISH))
+
+        val offset = OffsetDateTime.of(2021, 1, 6, 0, 0, 0, 0, ZoneOffset.UTC)
+        val actual = Localization.relativeTime(offset)
+
+        assertEquals("5 days from now", actual)
+    }
+}