Make TimestampedValue Parcelable

Make TimestampedValue Parcelable for simplicity.

TimetampedValue objects are not generally parcelable, depending on the
type of the value held. Previously, TimestampedValue did not implement
Parcelable to avoid committing to a general contract. Developers
parceling TimestampedValue objects were expected to call
TimestampedValue.writeToParcel() / TimestampedValue.readFromParcel()
explicitly when they knew it was safe to do so. This also meant that
TimestampedValues couldn't be used directly via AIDL.

This change makes TimestampedValue parcelable because it's more
familiar / convenient. Attempts to marshall a TimestampedValue that
contains a non-parcelable value will still throw a RuntimeException.

Bug: 140712361
Test: atest android.util.TimestampedValueTest
Change-Id: I8ca9c72f0433b380ce720cd813f650e743b3ddae