fix the bug that ect1 texture lost on android

[Canon0223]

• cocos2d-x version:3.9
• devices test on:android
• developing environments
  • NDK version:
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Steps to Reproduce:

#17278 not fix the bug
// etc1 support check whether alpha texture exists & load it
auto alphaFile = vt->_fileName + TextureCache::getETC1AlphaFileSuffix();
Texture2D::PixelFormat oldPixelFormat = Texture2D::getDefaultAlphaPixelFormat();
reloadTexture(vt->_texture->getAlphaTexture(), alphaFile, vt->_pixelFormat);

//After reload alphaTexture, you should setAlphaTexture,Like:
vt->_texture->setAlphaTexture(vt->_texture->getAlphaTexture());

//otherwise,ect1 texture will lost still