Avoid extra parsing in prepareContentForEdit()

* WikiPage::clear() no longer resets mPreparedEdit
* Also actually made mPreparedEdit protected

bug: 57026
Change-Id: I2c34baaf8b64b204444efa1bfa6ea356e0124c54
(cherry picked from commit a87989717086b114e0acf3353160063de1914114)