android set versionCode from git describe

-we need a monotonically increasing integer for android versionCode and
this works better across multiple branches

android/AndroidManifest.xml

 <?xml version="1.0"?>
-<manifest package="org.mavlink.qgroundcontrol" xmlns:android="http://schemas.android.com/apk/res/android" android:versionName="3.0.0" android:versionCode="2970" android:installLocation="auto">
+<manifest package="org.mavlink.qgroundcontrol" xmlns:android="http://schemas.android.com/apk/res/android" android:versionName="3.0.1" android:versionCode="301000" android:installLocation="auto">
     <application android:hardwareAccelerated="true" android:name="org.qtproject.qt5.android.bindings.QtApplication" android:label="-- %%INSERT_APP_NAME%% --" android:icon="@drawable/icon">
         <activity android:configChanges="orientation|uiMode|screenLayout|screenSize|smallestScreenSize|locale|fontScale|keyboard|keyboardHidden|navigation" android:name="org.qgroundcontrol.qgchelper.UsbDeviceJNI" android:label="-- %%INSERT_APP_NAME%% --" android:screenOrientation="sensorLandscape" android:launchMode="singleTask" android:keepScreenOn="true">
             <intent-filter>

tools/update_android_version.sh

 #!/usr/bin/env bash
 
-# this requires `origin/master` in the git tree
-
-MANIFEST_FILE=android/AndroidManifest.xml
-
-VERSIONCODE=`git rev-list origin/master --first-parent --count`
 VERSIONNAME=`git describe --always --tags | sed -e 's/^v//'`
 
+# Android versionCode from git tag vX.Y.Z-123-gSHA
+IFS=. read major minor patch dev sha <<<"${VERSIONNAME//-/.}"
+VERSIONCODE=$(($major*100000))
+VERSIONCODE=$(($(($minor*10000)) + $VERSIONCODE))
+VERSIONCODE=$(($(($patch*1000)) + $VERSIONCODE))
+VERSIONCODE=$(($(($dev)) + $VERSIONCODE))
+
+MANIFEST_FILE=android/AndroidManifest.xml
 if [ -n "$VERSIONCODE" ]; then
 	sed -i -e "s/android:versionCode=\"[0-9][0-9]*\"/android:versionCode=\"$VERSIONCODE\"/" $MANIFEST_FILE
 	echo "Android version: ${VERSIONCODE}"
@@ -22,3 +25,4 @@ else
 	echo "Error versionName empty"
 	exit 0 # don't cause the build to fail
 fi
+