Fix minimum Android API version and linking errors

[Robyt3]

The minimum supported API version must be specified when building the native libraries, otherwise this may cause linking errors when launching the app.

The minimum API level is increased to 24 (Android 7.0, covering 97.2% of usages) because:

• Vulkan is only available from API 24+ on ARM64 and x64.
• curl only compiles with API 23+.
• The NDK version we use supports only API 21+.

Ensure that the C++/Linker flags are set when building Android libraries, which was causing errors due to -fPIC not being set for all libraries.

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)