Allow notification strings to be unredacted in dump output.

This won't happen automatically and unredacted strings will
still not appear in bugreports, but if you are attached via
adb you can now do `dumpsys notification --unredact` to get
the contents of String/CharSequence fields and extras
emitted along with the rest of the dump.

The arg handling is also improved so that multiple filters
can be specified at once, e.g. --package (was "p") to
restrict to a single package, and --stats to get JSON
output.

Bug: 20451514
Change-Id: I37fc5ce86c7e28e8f8286917f6764d64bc081108