Create SdkLevelUtil for Mainline

Mainline modules should disable new APIs on older SDK levels
by checking the Build.VERSION constants.

Before S SDK finalization, checking
`VERSION.SDK_INT >= VERSION_CODES.S` doesn't work on S development
builds, because SDK_INT = R = 30, while VERSION_CODES.S = 10000.
Thus, check Build.VERSION.CODENAME as a workaround.

Bug: 154742276
Test: tested on S dev build
Change-Id: I51cb8136f6700fc93eb24f1941717b41d344dba4