[native_pdf_renderer] Transfer the bitmap through a temporary file

[graknol]

Is your feature request related to a problem? Please describe.
When rendering big PDFs or at big resolutions, the PlatformChannel takes a long time/crashes the app.

The PlatformChannel requires the UI thread, which means we shouldn't be transferring big files as base64-encoded JSON through it.

Describe the solution you'd like
At least the iOS and Android targets should create temporary files, then pass the path to this file in the response message.

Don't know how the web target works, so can't speak for that.

[graknol]

I'm taking a stab at the Android target.

[SergeShkurko]

#153

[SergeShkurko]

In you fork files location has broken: #153 (comment)

[SergeShkurko]

I resolve any problems at: #230
based on you changes but w/o commit history (i migrate files by hand)