Add flexibility on abiFilters.

context: https://github.com/FluidSynth/fluidsynth/pull/913#issuecomment-860195788

Run with `./gradlew -PcustomAbiFilters=x86` etc.

test-android/app/build.gradle

@@ -20,6 +20,9 @@ android {
                 cppFlags ''
                 arguments '-DANDROID_STL=c++_shared'
             }
+
+            if (project.hasProperty('customAbiFilters'))
+                ndk.abiFilters "$customAbiFilters"
         }
     }