Add secondary launcher mechanism (1/3)

Add a new config value for recording the component name of secondary
launcher.
This secondary launcher with corresponding launch mode set in
AndroidManifest could be used on secondary displays that support system
decorations.
OEMs can easily replace their own secondary launcher by overlay it.

Bug: 118206886
Bug: 111363427
Test: atest RootActivityContainerTests
Test: atest ActivityManagerMultiDisplayTests
Change-Id: Iceab096fd369127231f2085313ee617c7cdea226