Doc: Integrated the build instructions from the wiki

Reorganized the existing content to accommodate the build
instructions in the Qt5ForAndroidBuilding wiki page.

Task-number: QTBUG-33580
Change-Id: I49f54413cecdfa878a4271032ef38e028c6b315f
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>