AndroidContext will cause it to default to android storage engine

Also turn SQLiteStorageEngineFactory and create DefaultSQLiteStorageEngine factory with default behavior.

https://github.com/couchbase/couchbase-lite-android/issues/431
Use storage engine returned by context

No point in default fallback behavior since the Context interface forces returning a storage engine.

https://github.com/couchbase/couchbase-lite-android/issues/431
If cannot load file, return null for storage engine

https://github.com/couchbase/couchbase-lite-android/issues/431
Move JavaContext to couchbase-lite-java

Move JavaContext to couchbase-lite-java so that it can return its JavaSQLiteStorageEngineFactory
and as a result it doesn't need to use the default factory. The SQLiteStorageEngine text file can
be removed as well.
Remove DefaultSQLiteStorageEngineFactory

There is no need for the DefaultSQLiteStorageEngineFactory as couchbase-lite-java and couchbase-lite-android
have their own Context object which has access to its own SQLiteStorageEngineFactory/SQLiteStorageEngine.
Merge branch 'feature/issue_431_storage_engine'