android: allow to set a size of the thumbnail

Change-Id: Ida8bbd74893f59c3737d28c264e7ed24b231e6ac
android: call destroy only if the document exists

Change-Id: Ia9459151ac46a8be0f9a3944f2bcd427435c284e
android: clear all tiles on part change (method clearAllTiles)

Change-Id: I4e18fed5167559284bc18235811cb2b1c6c62a4d