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
Merged-In: I13eaccd20d9a6582c75b7f1ec2657b20205cd952
(cherry picked from commit 06b698c2)