Android build: set newer versions and parameterize ANDROID_PLATFORM

Hard coding desired ANDROID_PLATFORM on multiple places is simply bad.
Fix this. Further, set the variables to a much newer state.

CAVEAT: this will likely break android build, so be careful on
bisecting. All fixed in next, related commits.

Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>

android-mobile/AndroidManifest.xml

@@ -83,7 +83,7 @@
         </activity>
     </application>
 
-    <uses-sdk android:minSdkVersion="16"
+    <uses-sdk android:minSdkVersion="21"
 	      android:targetSdkVersion="26" />
 
     <supports-screens

packaging/android/build.sh

@@ -136,7 +136,7 @@ else
 fi
 
 if [ ! -e ndk-"$ARCH" ] ; then
-	"$ANDROID_NDK_ROOT/build/tools/make_standalone_toolchain.py" --arch="$ARCH" --install-dir=ndk-"$ARCH" --api=16
+	"$ANDROID_NDK_ROOT/build/tools/make_standalone_toolchain.py" --arch="$ARCH" --install-dir=ndk-"$ARCH" --api=$ANDROID_PLATFORM_LEVEL
 fi
 export BUILDROOT=$PWD
 export PATH=${BUILDROOT}/ndk-$ARCH/bin:$PATH
@@ -228,7 +228,7 @@ if [ "$QUICK" = "" ] ; then
 		make install
 		popd
 	fi
-
+	
 	"${SUBSURFACE_SOURCE}"/scripts/get-dep-lib.sh singleAndroid . libzip
 	if [ ! -e "$PKG_CONFIG_LIBDIR/libzip.pc" ] ; then
 		# libzip expects a predefined macro that isn't there for our compiler
@@ -399,7 +399,7 @@ cmake $MOBILE_CMAKE \
 	-DQT_ANDROID_SDK_ROOT="$ANDROID_SDK_ROOT" \
 	-DQT_ANDROID_NDK_ROOT="$ANDROID_NDK_ROOT" \
 	-DANDROID_TOOLCHAIN="gcc" \
-	-DANDROID_PLATFORM="android-16" \
+	-DANDROID_PLATFORM="$ANDROID_PLATFORM" \
 	-DCMAKE_TOOLCHAIN_FILE="$ANDROID_NDK_ROOT"/build/cmake/android.toolchain.cmake \
 	-DQT_ANDROID_CMAKE="$BUILDROOT"/qt-android-cmake/AddQtAndroidApk.cmake \
 	-DANDROID_STL="gnustl_shared" \

packaging/android/variables.sh

@@ -1,11 +1,13 @@
 #!/bin/bash
 # When changing Qt version remember to update the 
 # qt-installer-noninteractive file as well.
-QT_VERSION=5.11
-LATEST_QT=5.11.1
-NDK_VERSION=r14b
-SDK_VERSION=3859397
-ANDROID_BUILDTOOLS_REVISION=25.0.3
+QT_VERSION=5.12
+LATEST_QT=5.12.0
+NDK_VERSION=r18b
+SDK_VERSION=4333796
+ANDROID_BUILDTOOLS_REVISION=28.0.2
+ANDROID_PLATFORM_LEVEL=21
+ANDROID_PLATFORM=android-21
 ANDROID_PLATFORMS=android-27
 ANDROID_NDK=android-ndk-${NDK_VERSION}
 ANDROID_SDK=android-sdk-linux