Move LogBufferHelper to SystemUITestUtils.

This will allow us to use it in kosmos fixtures without needing to
add new dependencies.

Also moved it to the log package instead of dump since I felt it was a
better fit (and it saves some import statements).

Bug: 293167744
Test: builds
Flag: N/A

Change-Id: Ica0a45b1f593b47247eb4235554b5440b85b5df1