Revert "Use MAP_FIXED in remapNewHeap"

This reverts commit 462bc921aefe3b96e8cc7dae91e234b0e455966c, it
breaks android.openglperf.cts.GlAppSwitchTest#testGlActivitySwitchingFast

Change-Id: Ic153994859f678dbef4328bee5740e21beb735e0