Fix DecorView error about non-visual context

This error showed because DecorContext uses application context
to get WindowManager. This CL changes to use context to do so.
Also rename fields in DecorContext because we actually can pass any
context in "activityContext."
Note that most cases of misuse WindowManager is covered by [1].
We can guarantee that WindowManager can be obtained by mContext.

[1]:I52aa0c4a02b7da018aa10f1473e1616564296e41

Bug: 150632074
Test: manual - enable strict mode and check the error log not shown.
Test: atest DecorContextTest
Change-Id: I558a2819e5928a802b897a130cfc3262115b9935