ANDROID: drop KERNEL_DIR setting in build.config.common

And replace many references of ${ROOT_DIR}/common with
${ROOT_DIR}/${KERNEL_DIR}, which makes it a lot easier
to branch off of ACK and update the KERNEL_DIR.

The variable of KERNEL_DIR will be set in build/_setup_env.sh
to the directory of the build config file by default if it's
not set explicitly in the build config file or in environment.

Test: build test with following configs
        common/build.config.gki.aarch64
        common/build.config.db845c
        common/build.config.hikey960
        cuttlefish-modules/build.config.cuttlefish.aarch64

Bug: 162785964
Change-Id: I790ac0ded10bd790484f14c6f93d53e06c38b830
Signed-off-by: Will McVicker <willmcvicker@google.com>
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>