Export include dirs with -isystem

drm_property_type_is function in xf86drmMode.h file can cause compiler
error because it performs unsigned to signed conversion. Some Android.mk
modules have been avoiding this by adding "-isystem external/libdrm"
flag, because warnings from system headers are suppressed.

This changes exported_include_dirs to export_system_include_dirs
to workaround the potential error with the same manner above.

Bug: 157106227
Test: include xf86drmMode.h with -Wconversion
Change-Id: I13eaccd20d9a6582c75b7f1ec2657b20205cd952

Android.bp

@@ -44,7 +44,7 @@ cc_defaults {
         "-Wno-unused-parameter",
         "-Wno-unused-variable",
     ],
-    export_include_dirs: ["."],
+    export_system_include_dirs: ["."],
 }
 
 // Library for the device