Fix a bug in KeyStore.getInstance().

Sometimes keystore process may die unexpectedly, so client cannot connect it.

Let KeyStore.getInstance() return null when it cannot get KeyStore
service, so that caller can get informed to prevent NullPointerExcepion.

Change-Id: I7aafc959dee9a77fda80ab6eb5c82f9a3f56f959
Signed-off-by: hanpengx <pengfeix.han@intel.com>
Signed-off-by: Guobin Zhang <guobin.zhang@intel.com>