Improve memunreachable ABI before making it vendor_available

vendor_available: true means that some prebuilts will end up with
references to classes defined in memunreachable.h.  Add version
numbers and reserved fields to the classes and default initialize
everything to zero.

Bug: 132302484
Test: memunreachable_unit_test
Change-Id: Ic183fcc766acd2c4c7dc62efafcc2c75a636e407