base

Update javadoc for android.os.Bundle.

Call out the fact that defaultValue is returned if a null
mapping exists for a given key.

bug: https://code.google.com/p/android/issues/detail?id=68453

(cherry picked from commit ca2197b0457b1626b95a053d835ceaca43ac0286)

Change-Id: Iec70478ae091bb222be6e9ef9726c817395e9e23