[INTERNAL] Configuration: fix getter for full debug mode
[INTERNAL] Configuration: fix getter for full debug mode

Follow-up to I9209345588c68e2e81bae4671b49db4ac3b00dc9 which broke the
partial debug mode. Only a value of window["sap-ui-debug"] === true
should be reflected by getDebug. Otherwise, preloads will be suppressed
even for partial debug mode which renders the partial debug mode
useless.

The issue was not detected by the standard voters as they skip the unit
tests for debug mode (no debug files available in those voters).

Change-Id: I384c09209a77126f8d5ea150bbdf5c5791b7432a

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5