Define aconfig flags for settingslib

Adds a settingslib.aconfig that is separate from the existing
settlingslib_media_flag_declarations.aconfig file. The existing aconfig
uses a media package and it seemed to make more sense to break out a
shared settlingslib flag file

The new file defines com.settingslib.flags.new_status_bar_icons

Test: compile
Test: settingslib robo tests, settingslib unit tests
Bug: 314812750
Flag: ACONFIG com.settingslib.flags.new_status_bar_icons DEVELOPMENT
Change-Id: I33399555c81acaed314e2b8dc2fed37ef7249050