Fix app crash while running in second display

Launch DocumentsUI at second display,then crash happened after
pressing right button of mouse to trigger conext menu.

Create a new DecorContext instance for external display.

Bug: b/117215434
Test: atest PhoneWindowTest, DecorContextTest and manual test
Change-Id: I2f1dc1ac3721a20a7a49cfcde97fb168fcbfcc56