Dex-wide ArenaPool scoping for AOT compilation

The Arena Pool is a structure that holds memory allocated by the Arena
Allocator, preventing system allocations and deallocations to occur too often
during AOT compilation. Currently, the Arena Pool holds memory it allocates
during the whole AOT compilation process. Unfortunately, such a behavior
generates memory usage overhead, which is exacerbated by bigger applications
such as Facebook.

In this particular app, method size imbalance provokes unnecessary memory
pressure, as one method might require a lot of arena allocations that won't be
used in the remaining compilation. Because the compiler memory footprint keeps
increasing during AOT compilation, the memory pressure becomes very high.

The proposed patch is an attempt to find a tradeoff between allocations /
deallocations time overhead, and the aforementioned memory pressure overhead
resulting of the allocations being held by the Arena Pool. The patch adds a
feature freeing up all memory allocated in the Arena Pool after each dex file
is done compiling.

We have measured no significant AOT compile-time overhead with the patch
(<0.3%), because the selected dex-file granularity is coarse enough. In the
meantime, it provides significant memory footprint improvements. The impact is
especially big with Facebook, because this app has a few methods generating
huge memory footprint peaks (peak native heap footprint goes down 34%).

Change-Id: I27e867e6a20b8a6c28a82cb83140941a8c2b5847
Signed-off-by: Jean-Philippe Halimi <jean-philippe.halimi@intel.com>

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