Do not set default encoding to UTF-8 in KXmlParser

Manually setting the encoding makes kxml to ignore the BOM at the
beginning of the file and fail to parse some files. If null is passed,
kxml will check if there is a BOM and default to UTF-8 if there is none.

Bug: http://b.android.com/38055
Change-Id: I170d8fbb7567d2266f36a7768d4e63d9c2fa8286
(cherry picked from commit 89a5812b9eb8c62411a88f472468f3c978b8cfcd)