Create two sentinel api levels

This relands aosp/2470068. The original CL was submitted as part of a
stack that broke tm and udc. Those branches still contain soong modules
with min_sdk_version of type (kind+level).

Test: m nothing on tm and udc (via go/abtd)
Bug: 208456999

Change-Id: I8e013ec10530372f70f0ab0505b7eebeee2b360b

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",