Fix DisplayManagerGlobalTest

A few functions needed for tests were marked package protected
with VisibleForTesting annotation. However, the
DisplayManagerGlobalTests is unable to access these and requires us to make them public. Many other functions in DisplayManagerGlobal already follow this pattern

Bug: 391828526
Test: atest DisplayManagerGlobalTest
Flag: EXEMPT test fix
Change-Id: I7f66bba7e7d609ad72aed96927a0e3b7b64eaffa