Supports dynamic height for the new magnifier.

And also limits the pre-magnified line height:
 - If the text line height is too big, don't show the magnifier.
 - If the text line height is too small, enlarges the zoom factor so
   that the magnified text can be seen clearly.

Bug: 77791703, 148181186
Test: locally verified.
Change-Id: I4cf56e0a54fda6b23dafe153040fe5ba206f5f1f