Cleanup Android build and make compatible with ECM.

Change to proper use of CMAKE_SYSTEM_NAME, which is required by the
ECM toolchain. After this change, Marble-Maps can be build by the command:

cmake -DCMAKE_TOOLCHAIN_FILE=/path/to/ECM/toolchain/Android.cmake \
  -DCMAKE_PREFIX_PATH=/path/to/5.5/android_armv7/ \
  -DQT_NO_WEBKIT=TRUE -DQT_NO_PRINTER=TRUE -DQT_NO_DBUS=TRUE ../marble

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