Ensure looper is non-null in DisplayManagerGlobal

Looper.myLooper() can return null, according to the docs.
But in the DisplayManagerGlobal code, this method is assumed to return
non-null looper.

To ensure that a looper is available, use the main thread looper if the
looper for the current thread is null.

Test: atest cts.ViewGroupTest
Bug: 123592612
Change-Id: I80aaf5703e62837720395bfac4d95144cd6ab9f0