Fix compressible string check for ctors with `hibyte`.

Only the low 8 bits of the `hibyte` are relevant but we
were checking the whole `hibyte`, erroneously creating
an uncompressed string when the low bits are zero but
high bits are non-zero. String.equals() depends on all
compressible Strings being actually compressed.

Test: Added regression test to 021-string2.
Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 31040547
Bug: 63661357
Change-Id: I31690b544de928ce9c6f39f3d144d3e2029f38f6