Add support for profiling boot class path

Added a runtime option -Xps-profile-boot-class-path that makes the
profile saver take samples for the boot dex files. The motivation is
to use this for improving boot image creation.

Added test case to test 595.

Test: adb shell setprop dalvik.vm.extra-opts "'-Xusejit:false -Xint -Xps-profile-boot-class-path -verbose:profiler'"
Test: Pull profile and look at output
Test: test-art-host
Test: test/run-test --host 595-profile-saving

Bug: 37966211

Change-Id: I5c6681f307a7b8bde1cd88ca31578625d0651546