Android: fix some issues with jdk path detection

* There were two functions to detect jdk path, unified them.
* First try to find jdk 1.8, if not found, look for newer versions.
SDK Tools version 26.x needs jdk 1.8 however, the new cmdline-tools
can work with the newest jdk, so the UI will warn the user if the
selected jdk cannot run sdkmanager potentially because of the jdk
version.

Change-Id: Iee2c378598c26e8a9a8245262110ac20322a2d2b
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/android/androidsdkmanager.cpp

@@ -242,7 +242,7 @@ private:
     mutable QReadWriteLock m_licenseInputLock;
 
 public:
-    bool m_packageListingSuccessful = true;
+    bool m_packageListingSuccessful = false;
 };
 
 /*!
@@ -908,6 +908,7 @@ void AndroidSdkManagerPrivate::reloadSdkPackages()
     clearPackages();
 
     lastSdkManagerPath = m_config.sdkManagerToolPath();
+    m_packageListingSuccessful = false;
 
     if (m_config.sdkToolsVersion().isNull()) {
         // Configuration has invalid sdk path or corrupt installation.