Add class Context which wraps the Android/Java Context class and add a
JNI wrapper for method Context.getCacheDir().

Hard-coded path.  Will be replaced soon.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Finally, MPD runs on Android.  For some small value of "runs".  Very
much work left, too much to describe.

Fixes nullptr dereference (regression by commit df5f9f4a).

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

The new method IsPlugin() replaces the "is_simple" flag.

Use a listener interface instead of GlobalEvents.

Fix build failure.

Fixes nullptr dereference.

Build the UTF-8 version of the path automatically in the constructor.

Eliminate duplicate variable.

Move call from Mapper.cxx.

Move call from Mapper.cxx.

Don't use the obsolete Mapper library to create it.

Hide inside CreateLocalStorage().