misc/android: add build constraints on files intended to be built by filename only

Updates #30228

Change-Id: I91a763d94de935d9102d927b5cefee564bbf049b
Reviewed-on: https://go-review.googlesource.com/c/163208
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/android/cleaner.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Cleaner removes anything from /data/local/tmp/goroot not on a builtin list.
 // Used by androidtest.bash.
 package main

misc/android/go_android_exec.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // This program can be used as go_android_GOARCH_exec by the Go tool.
 // It executes binaries on an android device using adb.
 package main