Fix BSIC Check in CellIdentityGsm

Because Java uses signed bytes, a comparison between
a byte and 0xFF will always fail after integer
promotion. Fix this by forcing 0xFF to be treated as
a byte for comparison.

Bug: 119224773
Test: fixes build breakage

Change-Id: I9615667a6b442060f1d43084d56bc9c3fdb3ba35