Add system config to specify Multi-Display support

Added config_supportsMultiDisplay. Devices that have the config set
to false will not allow launching activities on secondary screens.

Bug: 36776777
Test: android.server.cts.ActivityManagerDisplayTests
Test: #testMultiDisplayDisabled
Change-Id: Iebb79044440866b1693897b0b53ccb987b5fd8e0
7 files changed