Fix DecorView error about non-visual context

This error showed because DecorContext uses application context
to get WindowManager. This CL changes to use PhoneWindow to obtain
WindowManager instance. Also refactor ctr to obtain context from
PhoneWindow.

Bug: 152806048
Test: manual - enable strict mode and check the error log not shown.
Test: atest DecorContextTest
Test: atest MemoryTests#testActivityRecreation
Change-Id: I1d416b9cdb015c9bc3553571041f3b14bb9da5da