Update to latest version

[MasterEmit]

I planed to check if I could build an updated version of pdfium but just replacing make files does not work anymore. Now I'm unsure what was changed in that make files last year to check if I the changes could be done with the new version too.
Is there any plan to update this repository to a pdfium version from 8.1 for example?

[ncruces]

PDFium was changed to use the Soong build system.

[xiasenhai]

i have tried to build a shared library on latest codebase but failed. it has troubled me for days。

[ncruces]

The latest version I was able to build was android-7.1.2_r36. Instructions here.

[xiasenhai]

@ncruces great!