ART: Fix memory unmapped twice issue in ElfFile::Load(bool)

Root Cause:
  The overlapped memory region will be unmapped by
  (1) ~MemMap() of reservation MemMap (reserve) and
  (2) ~MemMap() of "reuse" MemMap (segment).
  Someone takes the memory region after (1) and it will be unmapped in (2).
  So, SIGSEGV occurs when using the unmapped memory region.

Solution:
  Fixes this issue by skip unmap "reuse" MemMap in destructor.
  And always create reservation MemMap before "reuse" MemMap. (It also solved
  the fixupELF case which does not reserve the whole needed memory region).

Bug: 16486685

(cherry picked from commit a62a588a)

Change-Id: Icb83c8e87fa168027d9d8adb34925000399d3d2a