Hide MeasuredText.getChars

The MeasuredText.getChars is used only from text layout internals:
- LineBreaker#computeLineBreaks
- Canvas.drawtextRun

Thus, no need to be exposed to public. Making it @hide

Change-Id: I7c27a4b95d7d5ebc2ad5d74c947cdea3ce08f2ab
Fix: 124795470
Test: N/A
2 files changed