ui: Refactor stable ID generation for GPU virtual displays

This is a partial port of  http://ag/17832464 for main.

Use ftl::stable_hash to generate stable ID from uniqueId for GPU virtual
displays. This allows FLAG_STABLE consistent with PhysicalDisplayId that
uses a unique EDID, as well as being stable across reboots when using
hardcoded or static unique ID.

Add DisplayId.isStable()/isVirtual() interfaces and refactor previous
legacy usages.

Bug: 339525838
Bug: 137375833
Bug: 194863377
Test: atest libsurfaceflinger_unittest
Test: atest DisplayId_test
Flag: EXEMPT refactor
Change-Id: I54f4d3803c8c23266a3461660146af7ae017e4be