Use realActivity as indicator for task locking

When lock task mode is started, we verify that the package is
whitelisted and currently use mCallingPackage as indicator. However,
the calling package is not necessarily identical to the package trying
to lock itself, so lock task mode sometimes fails. Switching over to
using realActivity as package marker.

Bug: 22916291
Change-Id: Ifd4df2d634842c8106b0b0f690bcf1faba0ed5fa