Replace the resource flag to be used for google sans flex font changes

This new flag is going to be a bug fix flag for easier process
requirements.

Bug: 393609690
Test: Verified compilation worked well and font actually built
successfully in both gradle and make.
Flag: com.android.launcher3.gsf_res

Change-Id: Ic94e52e7fa58efb20fd867cfc661d6f7341fa4ef