[Android] Fix ImGui usage crash due to not clearing pointer

[rh101]

Describe your changes

This will fix the crash due to the assert statement in ImGui::Shutdown:

IM_ASSERT_USER_ERROR(g.IO.BackendRendererUserData == NULL, "Forgot to shutdown Renderer backend?");

The IO.BackendRendererUserData either contains the same value as IO.BackendPlatformUserData or NULL.

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