Verify number of Map entries written to Parcel

Make sure the number of entries written by Parcel#writeMapInternal
matches the size written. If a mismatch were allowed, an exploitable
scenario could occur where the data read from the Parcel would not
match the data written.

Change-Id: I325d08a8b66b6e80fe76501359c41b6656848607
Fixes: 112859604
Test: atest android.os.cts.ParcelTest