Add API to get the Android API level

The Android SDK version, or API level, is commonly used to determine
what APIs are available at runtime.

[ChangeLog][QtAndroidExtras] Added convenience method for getting the
Android SDK version.

Change-Id: Ic20322f68bc9680aec9e1c8404583462063904db
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>