Update min_sdk_version from SdkSpec to ApiLevel

min_sdk_version signifies device version and does not need an sdkKind to
describe it fully. Update the type and cleanup existing usages. As a
side benefit, we also get better error handling since users can no
longer enter something like `public_30` as a valid min_sdk_version in bp
files

Will do a similar cleanup for targetSdkVersion and maxSdkVersion in a
followup CL

Test: m nothing
Test: no change in ninja files (this should be a no-op)
Bug: 208456999

Change-Id: Ie6ae7e267d093c5e4787e82685daaca1021d202e
diff --git a/java/app_import.go b/java/app_import.go
index e24e780..c1de667 100644
--- a/java/app_import.go
+++ b/java/app_import.go
@@ -426,8 +426,8 @@
 	return android.SdkSpecPrivate
 }
 
-func (a *AndroidAppImport) MinSdkVersion(ctx android.EarlyModuleContext) android.SdkSpec {
-	return android.SdkSpecPrivate
+func (a *AndroidAppImport) MinSdkVersion(ctx android.EarlyModuleContext) android.ApiLevel {
+	return android.SdkSpecPrivate.ApiLevel
 }
 
 func (a *AndroidAppImport) LintDepSets() LintDepSets {