fix: can not delete embed blots on android

[wpj-wpj-wpj]

fix #1985

the reason of this issue

the cursor focus between the contentNode of the embed and the rightGuard of the embed which is a zero-width space

in this case, the native selection range is like

{
 start:{
   node: rightGuard,
   offset: 0,
 }
}

until now, everything is fine

but when quill normalize this range, it goes the wrong way, because offset is 0.

the final range we get is the position before our embed

so when you backspace

you can't delete the embed

you just delete the previous sibling

[mnordlindh]

This is the same fix as I proposed in my pull request #4464

Would be good if a maintainer could have a look at it.