Make per-display focus optional (1/2)

In general cases, we don't have multiple input methods simultaneously.
So that it may not make sense to have multiple focused window in the
system. Especially when there are multiple blinking cursors of input
boxes on different displays, the user may not be able to know which
input box can receive key events. In these cases, we let the system
has only one focused window.

In some cases, there can be multiple input methods simultaneously (one
for each display). To enable multiple users to interact with the
corresponding displays at the same time, we should let the system have
per-display focused window in these cases.

This CL makes per-display focus optional, and also reverts ag/5312657

Fix: 120241480
Fix: 120790099
Test: atest ActivityManagerMultiDisplayTests \
            CtsWindowManagerDeviceTestCases \
            WmTests

Change-Id: Ie030eed523599b217060887171710692d050e5d8