Replace the usage of deprecated APIs with newer ones

[friissoren]

When compiling, there are several warnings about using deprecated APIs. Figure out if it is possible to replace their usage with some APIs that are not deprecated.

C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\DebugActivity.java:186: warning: [deprecation] getExternalStoragePublicDirectory(String) in Environment has been deprecated
        File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS), "OpenDroneID");
                                        ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\DebugActivity.java:231: warning: [deprecation] setWifiEnabled(boolean) in WifiManager has been deprecated
                    wifiManager.setWifiEnabled(true);
                               ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\DebugActivity.java:338: warning: [deprecation] Handler() in Handler has been deprecated
        handler = new Handler();
                  ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\bluetooth\WiFiBeaconScanner.java:84: warning: [deprecation] setWifiEnabled(boolean) in WifiManager has been deprecated
            wifiManager.setWifiEnabled(true);
                       ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\bluetooth\WiFiBeaconScanner.java:182: warning: [deprecation] startScan() in WifiManager has been deprecated
        boolean ret = wifiManager.startScan();
                                 ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\DeviceList.java:233: warning: [deprecation] setColorFilter(int,Mode) in Drawable has been deprecated
                    droneIcon.setColorFilter(0xff00ff00, PorterDuff.Mode.MULTIPLY);
                             ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\HelpMenu.java:33: warning: [deprecation] fromHtml(String) in Html has been deprecated
        helpView.setText(Html.fromHtml(linkText));
                             ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\HelpMenu.java:43: warning: [deprecation] fromHtml(String) in Html has been deprecated
        helpView.setText(Html.fromHtml(linkText));
                             ^
C:\Android\receiver-android\Android\app\src\main\java\org\opendroneid\android\app\HelpMenu.java:50: warning: [deprecation] fromHtml(String) in Html has been deprecated
        helpView.setText(Html.fromHtml(linkText));
                             ^
9 warnings