Support for QLEService::writeDescriptor() on Android

Change-Id: I1c7f0491506c6f0512d097a419660c5f5e7fb144
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Fix wrong error signal emision on writeCharacteristic() on Android

There is no need to go around the event loop. Furthermore there is no
signal "void error(QLEHandle)" in QLEService. The signal is

void error(QLEService::ServiceError)

Change-Id: I53e9eb9b82cff38b15edab524329fa98206ab27b
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Apply fixes pointed out by Java code analyzer

This covers code optimizations and one minor corner case bug.

Change-Id: I9c4df9462e8610ea37a6e43e20840e537c295684
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Document and test concurrent write requests

BTLE communication is serialised. However the Qt API provides the
ability to issue multiple write requests while the first is still
pending. The backend must enqueue the requests for later on.

This behavior was already indirectly tests by a unit test but this
patch adds an explicit test section for it and documents the behavior.

Change-Id: I089b52940820bf0ba9c3ec872a6f1d5d6bd78a0e
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>