ANDROID: Creating boot-img.tar.gz for aarch64

The boot-img.tar.gz includes boot.img and boot-lz4.img,
with kernel image: Image and Image.lz4, respectively.

Bug: 222078981
Test: BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh
Signed-off-by: Bowgo Tsai <bowgotsai@google.com>
Change-Id: I7f929a73967ce87d0d653d0b9926198cfeedc973
(cherry picked from commit 3361d46a)