android: simplify the including rule of subdirs

Use android build system functions to include Android.mk
of subdirs.

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
[Emil Velikov: Resolve trivial conflicts.]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>