Add Manifest Merger support to Android module

This is needed to merge an application's AndroidManifest.xml with the AndroidManifest.xmls from dependent libraries. Especially useful for Qt itself, which needs to add additional content to a Qt-based APK's manifest file. Also, for handling injected attributes such that "${applicationId}" in an AndroidManifest.xml will be replaced with the value of the Qbs property product.packageName. Qbs should also be able to completely synthesize AndroidManifest.xml files similar to how Info.plist support is implemented for iOS and OS X.

https://android.googlesource.com/platform/tools/base/+/master/build-system/manifest-merger/src/main/java/com/android/manifmerger/Main.java

Usage: java -classpath $ANDROID_HOME/tools/lib/manifest-merger.jar com.android.manifmerger.Main merge --main main_manifest.xml --libs lib1.xml lib2.xml --out result.xml

We might want to make our own entry point to the manifest merger library and manually call ManifestMerger#process, to allow specification of injected attributes, since the existing command line wrapper doesn't support it. Not needed with Manifest Merger v2; there is a new entry point which supports injected properties and placeholders.