Android: Mark toolchains from not the current ndk as invalid

And ensure that they are removed on changing the ndk directory.

Change-Id: Ifd3de4d2ac53e93c70b683e11c1368755969ccee
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/android/androidsettingspage.cpp

@@ -83,7 +83,14 @@ void AndroidSettingsPage::apply()
             ProjectExplorer::ToolChainManager::instance()->registerToolChain(tc);
     }
 
-    // TODO deregister old automatic toolchains?
+    for (int i = 0; i < existingToolChains.count(); ++i) {
+        ProjectExplorer::ToolChain *tc = existingToolChains.at(i);
+        if (tc->type() == QLatin1String(Constants::ANDROID_TOOLCHAIN_TYPE)) {
+            if (!tc->isValid()) {
+                ProjectExplorer::ToolChainManager::instance()->deregisterToolChain(tc);
+            }
+        }
+    }
 
     AndroidConfigurations::instance().updateAutomaticKitList();
 }

src/plugins/android/androidtoolchain.cpp

@@ -101,7 +101,8 @@ QString AndroidToolChain::typeDisplayName() const
 
 bool AndroidToolChain::isValid() const
 {
-    return GccToolChain::isValid() && targetAbi().isValid() && !m_ndkToolChainVersion.isEmpty();
+    return GccToolChain::isValid() && targetAbi().isValid() && !m_ndkToolChainVersion.isEmpty()
+            && compilerCommand().isChildOf(AndroidConfigurations::instance().config().ndkLocation);
 }
 
 void AndroidToolChain::addToEnvironment(Environment &env) const