libbinder: BpBinder. move handle

note commit 85180c00
offset(BpBinder, mHandle) was exposed as API until mid 2019

but there is no real reason anyone should have used it (only really
needed for IPCThreadState/ProcessState/Parcel), so in order to simplify
some refactoring here, trying to move it to detect any breakages
early.

Bug: 167966510
Test: boot on many Pixel devices
Change-Id: Ie3eb90add4071615a31a947d2fcd3263613a4a48