staging: android: Reduce allocation time from system heap

If we are under heavy memory pressure the shrinker might be running on
the system heap. This causes contention with the page pool lock when we
try to allocate from system heap. To ensure that allocations aren't
slowed down by having to wait for the page pool mutex we will just try
to get the mutex. If we are unsuccessful we just bypass the pool
altogether.

Change-Id: I3e92e3081355a8528438e41e74b716a2b49a3891
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>