[MATLAB] `arrow.array.BooleanArray`'s `toMATLAB` method does not take slice offsets into account

[sgilmore10]

Describe the bug, including details regarding any error messages, version, and platform.

While working on #38415, I noticed that the toMATLAB method of arrow.array.BooleanArray does not take the slice offset into account. This will cause the toMATLAB method to return the wrong value.

Component(s)

MATLAB

[sgilmore10]

take

[sgilmore10]

The same bug also manifests itself when accessing the Valid property on all sliced arrays. This is because both BooleanArray's toMATLAB method and the Valid getter function call arrow::matlab::bit::unpack(const std::shared_ptrarrow::Buffer& packed_buffer, int64_t length). This function calls VisitBitsUnrolled(const uint8_t* bitmap, int64_t start_offset, int64_t length, Visitor&& visit) with the assumption that start_offset is always 0. However, when the array has been sliced, start_offset will not be 0. Updating our unpack function to accept start_offset as input should fix both issues.