how to turn off Sprite/Default's shader keyword "ETC1_EXTERNAL_ALPHA" in android?

Hi, in my project when I am using the frame debugger,
I found that all my sprite renderer's render passes has keyword ETC1_EXTERNAL_ALPHA turned on.
I am very sure that all my sprite atla(created by sprite packer) are ETC2 RGBA compressed.
(I understand that I can create ETC1 split alpha sprite atla,
by choosing ETC1 RGB compression for all sprites within a packing tag)
//-------------------------------------------------------------------------
the following 3 images are editor information when the game is running in editor
1.frame debugger(key word ETC1_EXTERNAL_ALPHA is on, but it should not)

2.frame debugger(becuase ETC1_EXTERNAL_ALPHA is on, a 1x1 default white tex is used)

3.sprite packer(this atla is ETC2 RGBA compressed, so keyword ETC1_EXTERNAL_ALPHA should not be ON when rendering)


what this ETC1_EXTERNAL_ALPHA keyword does in the actual shader, from UnitySprites.cginc:
it always sample a 1x1 white texture, and try to replace original sprite's alpha by the r channel of that split alpha texture(a 1x1 white texture), but "_EnableExternalAlpha" is set to 0 so it actually just doing nothing but wasting GPU fragment time.

Code (CSharp):

1. fixed4 SampleSpriteTexture (float2 uv)
2. {
3.     fixed4 color = tex2D (_MainTex, uv);
4.  
5. #if ETC1_EXTERNAL_ALPHA
6.     fixed4 alpha = tex2D (_AlphaTex, uv);
7.     color.a = lerp (color.a, alpha.r, _EnableExternalAlpha);
8. #endif
9.  
10.     return color;
11. }

//----------------------------------------------------------------------------
Questions:
a)Is this an intended behavior? or is ETC1_EXTERNAL_ALPHA always turned on because I am inside the editor but not an actual android device?

b)how can I turn this off? I just want to improve sprite renderer's performance in actual android devices -> "saving 1 tex2D() & 1 lerp()"

 

thanks,

does that mean I should
[solution A]
a1 write my new Sprite/Default shader, without ETC1_EXTERNAL_ALPHA defined
a2 create 1 singleton material using that shader in (a1)
a3.write an editor script that,
a3-1.detect every sprite renderer in scene & project
a3-2.if any sprite renderer don't have a material, assign my new singleton SpriteDefault material to it?
---------------------------------
or
---------------------------------
[solution B]
just write the shader and replace unity's Sprite/Default shader? (I am not sure if it is possible or how to replace it)