Fix heap verification and add runtime options.

Post GC heap verification is now enabled by default for debug builds.
Added four Xgc runtime options: preverify, nopreverify, postverify,
nopostverify.

Fixed another bug where the non-moving space could theoretically end
up outside of the card table address range.

Bug: 10672951
Change-Id: I5ba3cd96dbfb900263dba795242df74c563deaa3