SF: Cleanup creating / destroying virtual display APIs

Follow-up CL for cleanup requested by SF owner in http://ag/27202517.

Misc:
- Fix type conversion warnings for print format.
- Return status destroying virtual display in SF and add appropriate
checks in UT.
- Use static constexpr / const for compile-time effciciency.

Bug: 339525838
Bug: 137375833
Bug: 194863377
Test: atest libsurfaceflinger_unittest
Test: atest SurfaceFlinger_test
Flag: EXEMPT refactor
Change-Id: I7859720989c9244b26f8764c3323a8495064c888