Create two sentinel api levels

InvalidApiLevel:
This will be used for error handling if a user provided api level is
not recognized

PrivateApiLevel:
This will be used to differentiate the api level of sdk_version:"" from
sdk_version:"current" or sdk_version:"<active_codename>" (all used to be
FutureApiLevel previously). This was not necessary previously since the
type of min_sdk_version was SdkSpec(kind+level). Since it had access to
kind, it could check that it was not SdkSpecPrivate

Test: m nothing
Change-Id: I628b443c34bf2ec258d947dfec09f38b126bc6bb

android/sdk_version_test.go

@@ -37,11 +37,11 @@ func TestSdkSpecFrom(t *testing.T) {
 		},
 		{
 			input:    "_",
-			expected: "invalid_(no version)",
+			expected: "invalid__",
 		},
 		{
 			input:    "_31",
-			expected: "invalid_(no version)",
+			expected: "invalid__31",
 		},
 		{
 			input:    "system_R",