Android: Remove support for Qt < 5.4.0

The Android plugin has several places where the code branches off, depending on the Qt version being >= 5.4.0. It is unclear whether the < 5.4.0 code still works, but it is clear that it cannot be touched, and that it makes you feel uncomfortable when you want to fix something.

Task: Get rid of those checks, and set the minimum supported Qt version for Qt Android to 5.4.0.