libs/ui: Remove isStable() flag from DisplayId

There are currently no clients of DisplayId who use isStable().

This CL removes the parsing and serving of "stableness" of a DisplayId
by removing the isStable() API, and all associated tests.

See: go/edid-display-ids-al13

Flag: com.android.graphics.surfaceflinger.flags.stable_edid_ids
Bug: 352320847
Test: Display{Id|Identification} & libsurfaceflinger_unittest
Change-Id: I903fae86085001d32f77f34286c9f03ee481093b