Code contains multiple always true `byte <` checks

**Describe the bug**
The code contains multiple `x < ...` checks where `x` is a `byte` and the right-hand side is > 127 (= max `byte` value), so these conditions are always true:
- https://github.com/apache/drill/blob/5f8cdda2ac001d4267c3c903f3e92b9bf24e8713/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java#L92
- https://github.com/apache/drill/blob/5f8cdda2ac001d4267c3c903f3e92b9bf24e8713/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java#L116
- https://github.com/apache/drill/blob/5f8cdda2ac001d4267c3c903f3e92b9bf24e8713/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java#L127
- https://github.com/apache/drill/blob/5f8cdda2ac001d4267c3c903f3e92b9bf24e8713/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/StringFunctions.java#L1111
- https://github.com/apache/drill/blob/5f8cdda2ac001d4267c3c903f3e92b9bf24e8713/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/StringFunctions.java#L1260

Not completely sure what the fix here is, maybe `(byte & 0xFF) < ...`. And also for `StringFunctions` maybe it should be `128` (decimal) instead of `0x128` (hex, = 296 decimal); at least the `0x128` looks a bit suspicious.

Start with the five reported checks in exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java at lines 92, 116, and 127, and StringFunctions.java at lines 1111 and 1260. Determine the intended byte bounds, including whether StringFunctions should use 128 or 0x128, then verify that all five conditions no longer contain always-true checks and preserve the intended string behavior.

Written by the indexing model from the issue text.