convert key type to char[] to allow zeroing

Strings are immutable in Java, so the memory cannot be zeroed out. char[]
are very mutable, so they can easily be zeroed out after use.  Therefore
only accept the database key as a char[] to encourage people to wipe the
key from memory when they are done with it.

fixes #2254 https://dev.guardianproject.info/issues/2254
ignore AllowBackup lint warning, this is a library project

While it is possible to set this in a library project, it will conflict
with the setting in the app that uses this library.  Therefore it should
remain unset here, and leave that up to the app.
add blank <application /> tag to placate the test framework demons

Getting the tests to run reliably is a black art...