Set mDataSize properly in appendFrom

mDataSize only increases if we end up with mDataPos greater than
mDataSize.

Test: binder_parcel_fuzzer
Bug: 438098181
Flag: EXEMPT bug fix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ab883e120ae18ef93a4c6f17c05a14218da5118e)
Merged-In: I932ad50627d15452f67b39bd410d666dc8e70a42
Change-Id: I932ad50627d15452f67b39bd410d666dc8e70a42