Revert "Migrate libcore code to new classes libcore.util.Mutable{Int,Long}."

This reverts commit bb1ca695.

Reason for revert: Likely cause of the below error that causes the device to not
boot successfully. Reverting while I work out the reason.

"JNI DETECTED ERROR IN APPLICATION: attempt to access field int android.util.MutableInt.value from an object argument of type libcore
.util.MutableInt: 0x70339504"

Change-Id: I3b55cde24e3594c0fda8f0261a003f880c2869aa