Fix code example in Parcelable javadoc

DOCS ONLY

The code example for android.os.Parcelable contained several errors:

- There was no type parameter for Parcelable.Creator (due to unescaped
< > in the javadoc comment).

- There was no implementation of describeContents().

- The semicolon after the CREATOR declaration was missing.

This change fixes all of the above.

Fixes issue http://b/issue?id=2221120

Change-Id: Icaf932d079573cc7699f1caa643aac49e85ccca0