[MultiSplit] Avoid calling displayManager::getDisplayIds multiple times

This cl avoids uncessary calls to displayManager::getDisplayIds. We
should only call getDisplayIds once if the multi split flag is on.
Updating displayIds upon connecting/disconnecting is not supported at
the moment. This support will be added in the TODO bug.

Test: manual
Bug: 413550773
Bug: 393217881
Flag: com.android.window.flags.enable_multi_display_split

Change-Id: I5b88543078fe2f2d8bf4725d5dbd09d1e667adc8