Make emulated storage available via su.

Apps (e.g. file managers) expect that they get the same view of files in
su and non-su mode.

Change-Id: I613d021dddb7b90c34bd6d7cf6d2234448404731
Change target API from 17 to 18

Change-Id: I7adb61ad20740875e31a7e94b218aeb593e3b368
Fix several Lint's warnings

Change-Id: I8da5969d7fb2dd2bc2d0760e445da96470042598
su is an optional component

Do not build it unless explicitly included in PRODUCT_PACKAGES

Change-Id: Ic7337daeb61aa8eb862aa33c4533a0177db87aa1
Fix incorrect prerequisites

'Nothing to be done' build failed because

* $(LOCAL_MODULE) should not be a prerequisite
* The created symlink doesn't exist in the host out tree

Besides, all prerequisites of $(SYMLINKS) are removed. This is because
'make' checks the timestamp of the file the link references rather than
the link itself. Therefore $(LOCAL_INSTALLED_MODULE) is a useless
prerequisite. On the other hand, $(LOCAL_PATH)/Android.mk should not
be a prerequisite of $(SYMLINKS) unless it is also a prerequisite of
$(LOCAL_INSTALLED_MODULE). Otherwise, if $(LOCAL_PATH)/Android.mk is
newer than $(LOCAL_INSTALLED_MODULE), $(SYMLINKS) will always be
re-made.

Correct the prerequisites of $(SUPERUSER_RC) as well.

Change-Id: I5746e983046a9cdc3ee2b8ecd3827f1f7e6b596e