ipc: apr_vm: change the priority of apr_vm_cb_threa

When the CPU is very busy, the APR message may timeout
due to other high-priority threads blocking. Increase
the priority of apr_vm_cb_thread from 1 to 3 to unblock.

Change-Id: I11be527b270e55dccafa67ed4c7b8b25799b8542
Signed-off-by: Hongtao Peng <hongtaop@codeaurora.org>