Unify context in ViewRoot

To fix the issue that mContext is not updated when moved to another
display, we previously introduce mDisplayContext as a workaround in [1].
Change to use Context#updateDisplay(int) to unify mDisplayContext to
mContext.

[1]: Ifa0ca24e9cf764caf492ee065606680b6a06751a

Bug: 116349163
Change-Id: I3adbb32a584ac3405563904c1fd077e96e0807c1
Test: atest ActivityManagerMultiDisplayTests
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest InputMethodManagerTest
Test: atest InsetsControllerTest
Test: atest InsetsSourceConsumerTest
Test: atest ImeInsetsSourceConsumerTest
Test: atest CtsActivityManagerDeviceTestCases:MultiDisplayClientTests