libbinder_ndk: AParcel_(get|set)DataPosition

This allows you do the following:
- a = getPosition
- writeInt32(0)
- b = getPosition
- write data of unknown size
- c = getPosition
- setPosition(a)
- write(c - b)

This allows to skip reading the data of unknown size.

Bug: 118395170
Test: atest android.binder.cts
Change-Id: Id8033705331c6a48327c240ad33bd2d5f9fc190c
3 files changed