floss: Add install target to build.py

The install target will copy binaries to target directory and strip
binaries so that they're smaller. Also add an option to not strip
binaries during install (default is to run strip).

Bug: 207009162
Tag: #floss
Test: ./build.py --target install
Change-Id: Ifb805a35623e4243cb28e63478a06ae0747c579f