Reduce memory lost by ArenaAllocator for large allocations.

When allocating from a new arena, check if the old arena has
more remaining space than the new one after the current
allocation. If so, keep using the old arena to reduce the
amount of "lost" arena memory. This can happen when we try
to allocate more than half the default arena size. If the
allocation exceeds the default arena size, it's very likely
to happen even though the ArenaPool could still provide some
much larger previously allocated arena.

Also avoid artithmetic overflow when checking if the
request can be satisfied from the current arena.
And abort immediately if calloc() fails.

Bug: 28173563
Bug: 28256882

In addition to the initial CL
    (cherry picked from commit 3e0e7173)
this contains a squashed subsequent fix
    Fix valgrind tests: mark allocated space as defined.
    (cherry picked from commit 3f84f2cb)

Change-Id: Id80d5601874e8e28d930c0dd47a51c73c4810094