Do not rely on peeking into toolchain files to detect an Android build

To figure out that we're doing an Android build we're peeking into the toolchain file on every platform (see qt_auto_detect_android). That is done to set certain default variables for Android, before the toolchain file is loaded.

The current approach is bound to break should the magic string we're looking for ever change.

Find some other means to detect whether we're attempting an Android build.