android: Put document into center when it's smaller than viewport

Change-Id: I0d0cd421e366bf34b71d9d82dd21476ecfa90b18
android: Tweak initial zoom of documents in different situations

- In case of Writer document always show document by full width.
- Otherwise show the whole document on the screen even when the
 orientation is portrait or landscape.

Change-Id: Ia6704432c850a5487aee1693ed12e039ae6ef0cb