Init: Run boringssl self test via separate binaries.

Instead of init.cpp knowning about the boringssl self
test, use init.rc to exec dedicated self test executables.

Advantages:
 - The self test is run not only both the copy of libcrypto
   in /system but also /apex/com.android.conscrypt.
 - The self test is run not only for the primary (e.g. 64bit)
   ABI but also for a secondarry (e.g. 32bit) ABI.
 - The dependency on libcrypto is kept to the self test binary.
 - The self test binary abstracts the exact native API for
   running the self test (this will change soon because the
   self test will be run when the library is loaded).

Bug: 137267623
Test: Check that logcat shows both binaries being started as root,
      and finishing with exit code 0.

Change-Id: I1e716749ee2133993f0f7b2836483391fd1a62f0