Update the default platform version to S

R is out. The platform codename on the main branch is now S and the API
level of the platform is 30, which is R because S hasn't been finalized.

Bug: N/A
Test: rm -rf out; ./build/soong/soong_ui.bash --make-mode --skip-make
Change-Id: I486b9639bccfe17a42681e1f84097373f8f401e7