libhwui: make surface buffer allocation asynchronous

On the critical path of the cold launch of applications
the main thread of the started application allocates
the surface buffer. The allocation is synchronous and blocks
the main thread of the application.
As a consequence the launch time of the application is delayed
by the time spent doing the allocation.

With this optimization the allocation is performed
asynchronously in the RenderThread. This optimization
will benefit to the launch of all applications.

Change-Id: I4bc145cfc3ba6fe1efbca519bcee2e4ea6617ae7
Signed-off-by: Thomas Buhot <thomas.buhot@intel.com>
Signed-off-by: Zhiquan Liu <zhiquan.liu@intel.com>