Script to build sdm845.

Eventually this script will be used instead of
http://google3/wireless/android/keystone/android-build-container/build-p-keystone-qcom-sdm845.sh.

Bug:77818247
Test: make -j droid showcommands dist platform_tests
Change-Id: I3ed0cb5fa838202d698eacd47e4aaa41167fa8fa