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PrepareWriteValue is a procedure to reliably write to GATT
characteristics in a GATT service. A client can PrepareWrite to
various characteristics in a reliable write session and Execute or Abort
those writes later.

For now, BlueZ handles prepare writes by caching them and then sends out
regular write requests to the application in the case of commit, or
silently drops them in the case of abort.

We keep the caching in BlueZ but implement a new function named
PrepareWriteValue() such that applications can tell prepare write
requests from regular write requests. Since BlueZ only sends out the
write requests when the remote device commits, applications not
interested in achieving reliable writes can safely regard all prepare
write requests as regular ones.

A relevant patch is crrev.com/c/1094214

Bug: b:78650442
CQ-DEPEND: CL:1094214
Test: Pass Android CtsVerifier test on GATT server reliable write
Change-Id: If2464cf3a17e7c91dc11dc8f4c37734a2987543e
Reviewed-on: https://chromium-review.googlesource.com/1094316
Commit-Queue: Qiyu Hu <qiyuh@google.com>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Miao-chen Chou <mcchou@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573539}