Set core platform API policy from command line flag

So as to avoid any performance penalty from core platform API access
checks, disable them by default. AndroidRuntime.cpp now passes the value
of the "persist.debug.dalvik.vm.core_platform_api_policy" to the
runtime, allowing a user to enable core platform API checks on demand.
The workflow for a tester is:
- adb shell setprop <property> "just-warn"
- adb reboot
- adb logcat | grep "Core platform API"

Test: manual, observe warnings in logcat
Test: art/test/testrunner/run_build_test_target.py art-test
Bug: 125701194
Merged-In: Iae09a8f44918dc349bc10dbdd703043667f51268
Change-Id: Iae09a8f44918dc349bc10dbdd703043667f51268
(cherry picked from commit acbd490663dbe4750834cafdde016831632367b7)