Make TextAppearanceInfo support spannable text

Extract the text appearance from the character before cursor when its TextPaint is overided by CharaterStyle spans. Also move the assignment code from Editor to TextAppearanceInfo#createFromTextView for easier testing.

Bug: 261378534

Test: atest android.view.inputmethod.TextAppearanceInfoTest

Change-Id: I9365767762bce5137f454260e342e833d3f35e55