IllegalStateException if apex is unavailable.

apexservice is expected to be active before zygote is run. In the
exceptional case it isn't we should fail fast and throw a clear error
message. The current version of the code would just assign "null" to the
mApexService variable, thus causing a null pointer exception later in
the code.

Bug: 124363450
Test: device boots.
Change-Id: I07361163c6e9efd392a633b30e85ca6be9f58ab3