ANDROID: Adding Image.gz and boot-gz.img

This adds <10 seconds for a LTO=thin build, while the original
build time is about ~250 seconds on a build machine with 48 2.60 GHz
processors and 64GB RAM.

Within the 10 seconds, ~3 seconds spent on building Image.gz,
~5 seconds for packing Image.gz as boot-gz.img and <1 second for
including the boot-gz.img into the boot-img.tar.gz.

Bug: 233352819
Test: LTO=thin BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh
Signed-off-by: Bowgo Tsai <bowgotsai@google.com>
Change-Id: I283ef004d1233d2e175573b893379e0d68221353