Prefetch standalone system server jars before switching domain.

In b/203198541, we start to compile standalone system server jars, but
the SELinux policy doesn't allow system_server to load the compiled code
at runtime. As a solution, this change creates class loaders for
standalone system server jars and caches them before switching to the
"system_server" SELinux domain, which essentially loads the jars and
compiled code into the memory.

Bug: 203198541
Test: manual -
  1. Patch aosp/1874113, aosp/1876173, aosp/1906158 (in order to have
     odrefresh compile a standalone system server jar).
  2. Build a system image and flash it to a device.
  3. Run "adb shell cat /proc/`adb shell pidof system_server`/maps" and
     see "apex@com.android.wifi@javalib@service-wifi.jar@classes.odex"
     being mmap'ed.
  4. Run "adb logcat | grep avc" and see no denial.
  5. atest odsign_e2e_tests

Change-Id: I365be766cfb77c6e240d5871f1b3d8ed7abae6f5