Use cleared JNI weak sentinel from boot image.

We were already adding the sentinel to the boot image,
so we may as well reuse the boot image copy.

Also move pre-allocated objects from class roots to the
boot image live objects.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: aosp_taimen-userdebug boots.
Change-Id: I635dcdd146ca2c6b55d187e9a545a9990b0b35ca
9 files changed