set NDK_PLATFORM_LEVEL & NDK_TOOLCHAIN_VERSION only in jni/Application.jni

This makes the external/Makefile get the values of these two variables from
the official Android place to set them, keeping the build systems in sync.

NDK platform level, aka APP_PLATFORM, is equivalent to minSdkVersion.

http://stackoverflow.com/questions/14551113/android-ndk-multiple-api-levels
set the NDK APP_PLATFORM based on minSdkVersion in AndroidManifest.xml

This way we avoid annoying, arcane build errors caused by different parts
of the build being built for different platform levels.
document the requirement for deps to be installed on the build system
update to sqlcipher v2.2.1
handle init errors when mounting

requires latest changes from libsqlfs