Fix ImmutableEnum lint (#47739)

This was introduced in #47315. Internally, this lint breaks the build with the following error:

```
shell/platform/android/io/flutter/embedding/android/KeyData.java:78: Error: DeviceType is an enum, which should be immutable, but field DeviceType.value is not final [ImmutableEnum]
    private long value;
    ~~~~~~~~~~~~~~~~~~~
```

See also https://errorprone.info/bugpattern/ImmutableEnumChecker.

Fixes: b/309552840

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/platform/android/io/flutter/embedding/android/KeyData.java

@@ -75,7 +75,7 @@ public enum DeviceType {
     kJoystick(3),
     kHdmi(4);
 
-    private long value;
+    private final long value;
 
     private DeviceType(long value) {
       this.value = value;