Prevent CertInstallerMain from being recreated upon screenSize change

Steps to Reproduce on Nexus 10:
1.Enter Settings app->Wi-Fi.
2.Tap options menu and select "Advanced" item.
3.Tap "Install certificates" item.
4.Rotate the device.
5.Two instances of DocumentsActivity will be created.

In step 3, Settings app will start CertInstallerMain Activity(invisible)
when user want to install certificates, then CertInstallerMain
will start DocumentsActivity with action Intent.ACTION_OPEN_DOCUMENT to
open document in its onCreate() method. When user rotate the screen, the
configuration screenSize will change, then the CertInstallerMain
will be destroyed and recreated, the DocumentsActivity will be created
again in onCreate() method of CertInstallerMain.

Change-Id: I0d31b9936aa158afbea6f426b5216fad50e6b237
Signed-off-by: hanpengx <pengfeix.han@intel.com>
Signed-off-by: Guobin Zhang <guobin.zhang@intel.com>