OtherCommands: split handle_update()
db/Interface: add virtual method Update()

For database plugins that don't use the UpdateService.
db/proxy: implement Update()
Makefile.am: new zipalign path
Makefile.am: release .apk depends on icon

Missing piece from commit e0ca4347
Makefile.am: move duplicate specification to APK_DEPS
Makefile.am: *.apk depends on android/src/*.java

Invoke "ant" when a Java source is modified.
android/Loader: fix indent
android/Main: indicate when the native code has quit

Let the user know that MPD has failed.  Not the best thing to do, but
better than pretending it still runs.
Main: don't require mpd.conf on Android

Don't fail to start if mpd.conf does not exist; just use default
values.
release v0.18.12
Merge tag 'v0.18.12'
Makefile.am: add missing backslash

Fixes breakage by commit 1a619522
android build 5
playlist/extm3u: use nullptr instead of NULL
pcm: use nullptr instead of NULL
system/Resolver: use nullptr instead of NULL
encoder/opus: use nullptr instead of NULL
StateFile: use nullptr instead of NULL
util/DynamicFifoBuffer: fix typo in API documentation