Zygote: Fix an issue when empty the usap pool.

When empty the usap pool, the usap processes may be blocked by
usapPoolSocket.accept(), which cause the usap process fail to kill
immediately. So we can move the position of blockSigTerm after
usapPoolsSocket.accept() and delele the old blockSigTerm in the while
loop.

Test: manual test.
1.setprop persist.device_config.runtime_native.usap_pool_enabled true.
After 1 min, trigger fill usap pools.
2.setprop persist.device_config.runtime_native.usap_pool_enabled false.
After 1 min, trigger empty usap pools.
3.repeat step 1.

Signed-off-by: zhangjianqiu <zhangjianqiu@oppo.com>
Change-Id: I657940b30f71cdc717c673be6d70738e61e2bb68