Update target_sdk_version from SdkSpec to ApiLevel

target_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 target_sdk_version in bp
files

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

Change-Id: I3c19245e29184bd9e5660ad8981966f64dfa9424
diff --git a/android/sdk_version.go b/android/sdk_version.go
index 2107cbe..26e002a 100644
--- a/android/sdk_version.go
+++ b/android/sdk_version.go
@@ -31,9 +31,9 @@
 	// ReplaceMaxSdkVersionPlaceholder returns SdkSpec to replace the maxSdkVersion property of permission and
 	// uses-permission tags if it is set.
 	ReplaceMaxSdkVersionPlaceholder(ctx EarlyModuleContext) SdkSpec
-	// TargetSdkVersion returns the SdkSpec that corresponds to the target_sdk_version property of the current module,
+	// TargetSdkVersion returns the ApiLevel that corresponds to the target_sdk_version property of the current module,
 	// or from sdk_version if it is not set.
-	TargetSdkVersion(ctx EarlyModuleContext) SdkSpec
+	TargetSdkVersion(ctx EarlyModuleContext) ApiLevel
 }
 
 // SdkKind represents a particular category of an SDK spec like public, system, test, etc.