Rename Version::ANDROID_* to kVersionFeatureLevel* -- hal

Prior to this change, version constants (e.g., Version::ANDROID_S) were
public static constants to make the version constants look as if they
were enum values. However, this method prevented versions from being
constexpr, because the Version type was incomplete by that point in
time. This change moves these version constants outside of the Version
struct, and makes them constexpr. They have the new names:
* Version::ANDROID_OC_MR1 -> kVersionFeatureLevel1
* Version::ANDROID_P -> kVersionFeatureLevel2
* Version::ANDROID_Q -> kVersionFeatureLevel3
* Version::ANDROID_R -> kVersionFeatureLevel4
* Version::ANDROID_S -> kVersionFeatureLevel5
* Version::FEATURE_LEVEL_6 -> kVersionFeatureLevel6
* Version::EXPERIMENTAL -> kVersionFeatureLevelExperimental

Bug: 206975939
Test: mma
Change-Id: Ibf5f2fdb1459a69c51865aa5fdcd0cb0c3a88ade