build.gradle: Allow debug builds for armv8

Otherwise we get a nasty libflutter.so missing error when running a
debug build on an actual armv8 device.

android/app/build.gradle

@@ -75,7 +75,7 @@ android {
         }
         debug {
             ndk {
-                abiFilters 'armeabi-v7a', 'x86', 'x86_64'
+                abiFilters 'arm64-v8a', 'armeabi-v7a', 'x86', 'x86_64'
             }
         }
     }