Add helpful options to skip build/package/strip.

This change adds the following flags:
--skip-build / -sb : Turn off building (only packaging).
--skip-package / -sp : Turn off packaging (only building).
--no-strip : Turn off stripping of binaries/libraries.

Test: Ran toolchain/llvm/android/build.py with/without new flags.
Change-Id: I72d025f3701f102f5b2cd42405c88a8eab089161