config_supportsMultiWindow for device to specify multi-window support

Devices that have the config set to false will not allow any multi-window
operation.

Also, added ActivityManager.supportsMultiWindow that checks the new config
and also returns false if the device is a low RAM device.

Bug: 27419483
Change-Id: I8dd85c17d290a5a752de3253beb3b34c17d7736d