selftests/android: initialize heap_type to avoid compiling warning

Initialize heap_type to ION_HEAP_TYPE_SYSTEM to avoid "used uninitialized"
compiler warning. heap_type gets used after initialization, this change is
to just keep the compiler happy.

root@vm-lkp-nex04-8G-7 ~/linux-v4.18-rc2/tools/testing/selftests/android# make
make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
make[1]: Entering directory '/root/linux-v4.18-rc2/tools/testing/selftests/android/ion'
gcc  -I. -I../../../../../drivers/staging/android/uapi/ -I../../../../../usr/include/ -Wall -O2 -g    ionapp_export.c ipcsocket.c ionutils.c   -o ionapp_export
ionapp_export.c: In function 'main':
ionapp_export.c:91:2: warning: 'heap_type' may be used uninitialized in
this function [-Wmaybe-uninitialized]
  printf("heap_type: %ld, heap_size: %ld\n", heap_type, heap_size);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

CC: Shuah Khan <shuah@kernel.org>
CC: Pintu Agarwal <pintu.ping@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>