[Android] increment shared_ptr for jni impl.

[jonahwilliams]

More speculative fixes for crashing. Since we were closing over a ref, maybe it is nullptr? Lets check with an FML_CHECK too.

[flutter-dashboard]

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[auto-submit]

autosubmit label was removed for flutter/flutter/163152, because - The status or check suite Windows build_tests_3_9 has failed. Please fix the issues identified (or deflake) before re-applying this label.