Android: Fix for build issue on x86

In the toolchain for x86 the va_list type is defined as char *, which
in itself isn't strange, but it was somewhat unexpected as it differs
from the arm toolchains. Either way we should not make assumption about
the va_list type as there is no guarantee it won't cause conflicts when
overloading. This fix simply renames the private overloads.

Change-Id: I7808619d0fa3ca63b75796308cfdff6aa41a7fd0
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>
Doc: updated documentation QString::toDouble

Task-number: QTBUG-35543
Change-Id: I60d5cc253e6d6a24e9b031758e16a547a9a07443
Reviewed-by: Martin Smith <martin.smith@digia.com>