Implement compatibility support for WRITE_SDCARD permission.

Now old applications will automatically be granted it.  Also renamed it from
SDCARD_WRITE to WRITE_SDCARD to be consistent with our other permissions,
and re-arranged how we do targetSdkVersion to actually be usuable for this
kind of stuff.

Note that right now this results in basically all apps being given the
WRITE_SDCARD permission, because their targetSdkVersion is not set.  I will
be dealing with that in a future change.