Make the default java_sdk_library dist_group "unknown"

Change the default dist_group from "android" to "unknown" to prevent
accidentally including java_sdk_library stubs that do not set
dist_group or owner in the public SDK.

Bug: 186723288
Test: TestJavaSdkLibraryDist
Change-Id: I9aae2a16254ac1a8d444acfa63bc571d1ef4b045
Merged-In: I9aae2a16254ac1a8d444acfa63bc571d1ef4b045
(cherry picked from commit 3dd662509d)

java/sdk_library_test.go

@@ -888,7 +888,7 @@ func TestJavaSdkLibraryDist(t *testing.T) {
 	testCases := []testCase{
 		{
 			module:   "sdklib_no_owner",
-			distDir:  "apistubs/android/public",
+			distDir:  "apistubs/unknown/public",
 			distStem: "sdklib_no_owner.jar",
 		},
 		{
@@ -903,7 +903,7 @@ func TestJavaSdkLibraryDist(t *testing.T) {
 		},
 		{
 			module:   "sdklib_stem_foo",
-			distDir:  "apistubs/android/public",
+			distDir:  "apistubs/unknown/public",
 			distStem: "foo.jar",
 		},
 		{