Update SdkLevel and sdk_level.h

Ideally, we would deprecate ApiLevelUtil classes maintained by tradefed
in favor of these utils. Since tradefed drops the prebuilts / sources
into old branches, we should avoid using fixed VERSION_CODES in code.

This makes the util behave similarly to BuildCompat which
understands pre-release builds as well. See https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:core/core/src/main/java/androidx/core/os/BuildCompat.java.

Bug: 187807290
Test: m
Change-Id: I4f1e34759218df62affbefbe1c6db8782152f033