apex: checks min_sdk_version for preview/current

If we don't check "current", it won't be checked even in the finalized
branch.

If we don't check "preview", it should be done during the SDK
finalization. It'd be better done before the SDK finalization regarding
that setting min_sdk_version is to get approval from deps library owners.

Bug: 177833148
Test: m (soong tests)
Change-Id: I712b61cfe5a134fbb69c73956d26fb3a1e5c011e
diff --git a/android/api_levels.go b/android/api_levels.go
index 08328e1..1b53f3f 100644
--- a/android/api_levels.go
+++ b/android/api_levels.go
@@ -81,6 +81,10 @@
 	return this.value == "current"
 }
 
+func (this ApiLevel) IsNone() bool {
+	return this.number == -1
+}
+
 // Returns -1 if the current API level is less than the argument, 0 if they
 // are equal, and 1 if it is greater than the argument.
 func (this ApiLevel) CompareTo(other ApiLevel) int {