Skip spinlock overhead on Android

Using the spinlock is only necessary when we multiple threads
might use a GrContext.  Android uses the GrContext from a
single thread, so these locks are not needed.

This is a temporary fix until we can refactor to avoid
creating GrContexts in a global memory pool.

BUG=skia:5696
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2293633003

Review-Url: https://codereview.chromium.org/2293633003
2 files changed