frameworks/base: Fix memory leak in BitmapRegionDecoder

When un-supported image is passed to bitmap decoder, mStream object
of SkMemoryStream class is leaking its resources. This is getting
called frequently when gallery app is trying to render thumbnails
in album view and due to this frequent sigkills were seen on
gallery3d app with adj 0 value

CRs-Fixed: 551617
Change-Id: I1d9401310566b529d81b406213f4e37be3104d95