Revert "Revert "Set mmap_rnd_bits to maximum value.""

This reverts commit c8f026fc.

The above revert was meant only for AOSP, not internal master, since the
kernel prebuilts  were not yet available there.  Revert the revert
internally so that internal builds, which have the appropriate kernel
prebuilts, operate properly.

Bug: 27681085
Change-Id: Ide81d43cd22e37979c8266342677db896da1c792