ANDROID: GKI: scripts: Makefile: update the lz4 command

Use 'lz4' instead of 'lz4c' and increase the compression level. This
also optimizes for decompression speed at this higher level.

Bug: 159285792
Test: launch_cvd -kernel_path bzImage
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: I9604cc7d1f3600dec27ceb460f7c29d977a919b1