ZygoteInit: warm up JCA providers during preload

This makes the time spent in the first call of an app to
SSLSocketFactory.getDefault() drop from ~240 ms to ~50 ms. In M
it was around ~6ms. This is due to the fact that, while instantiating
the default factory, all providers are initialized.

In order to obtain the timings above, I created an app calling
SSLSocketFactory.getDefault in onCreate and timed it
with System.currentTimeMillis() .

Bug: 28545496

Change-Id: I650d4b0435e67e481a41e02b0b538ce5cc993fa3