Explicit public visibility, SinceKotlin and ExperimentalTime status

Use file level ExperimentalTime opt-in in tests.

libraries/stdlib/jvm/src/kotlin/time/DurationUnitJvm.kt

@@ -8,8 +8,12 @@
 
 package kotlin.time
 
+@SinceKotlin("1.3")
+@ExperimentalTime
 public actual typealias DurationUnit = java.util.concurrent.TimeUnit
 
+@SinceKotlin("1.3")
+@ExperimentalTime
 internal actual fun convertDurationUnit(value: Double, sourceUnit: DurationUnit, targetUnit: DurationUnit): Double {
     val sourceInTargets = targetUnit.convert(1, sourceUnit)
     if (sourceInTargets > 0)

libraries/stdlib/jvm/src/kotlin/time/MonoClock.kt

@@ -5,6 +5,8 @@
 
 package kotlin.time
 
+@SinceKotlin("1.3")
+@ExperimentalTime
 public actual object MonoClock : AbstractLongClock(unit = DurationUnit.NANOSECONDS), Clock { // TODO: interface should not be required here
     override fun read(): Long = System.nanoTime()
     override fun toString(): String = "Clock(System.nanoTime())"