imageio: add limited PNM support for reading

see: http://netpbm.sourceforge.net/

Only reads P5 and P6 pnm files for now.

Change-Id: I2332a623f803df67455047f570f1cff9f464480a

build.gradle

@@ -288,6 +288,7 @@ model {
             include "jpegdec.c"
             include "metadata.c"
             include "pngdec.c"
+            include "pnmdec.c"
             include "tiffdec.c"
             include "webpdec.c"
           }