Use DisplayContext to obtain InputMethodManager in ViewRootImpl

Historically ViewRootImpl#mContext has been just pointing to the
Context instance associated with whatever first View instance that is
being added to a Window.  In other words, ViewRootImpl#mContext is not
owned by ViewRootImpl.

This can become a problem if ViewRootImpl is moved from a display to
another display with ViewRootImpl#updateInternalDisplay(), which was
added last year to support multiple displays [1], because there is no
guarantee that the target Contaxt that is pointed from
ViewRootImpl#mContext is also updated to have the new display ID.

In order to support multi-display we need to use the right Context at
least when interacting with InputMethodManager. Hence as a short term
solution this CL introduces ViewRootImpl#mDisplayContext so that the
right instance of InputMethodManager can receive callback from
ViewRootImpl. In this way, we can make sure that the behavior change
in ViewRootImple is limited to how InputMethodManager instance is
obtained.

Longer term solution will be discussed in Bug 116349163.

 [1]: I9a9501cab788623ada15a31efb53e4b2378639fe
      b047b8bd

Bug: 115893206
Test: atest ActivityManagerMultiDisplayTests
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Ifa0ca24e9cf764caf492ee065606680b6a06751a