Deduplicate StickyClass GC roots

[pyricau]

System classes are GC roots and tracked by GcRoot.StickyClass. We've seen heap dumps from API 23 emulators that included millions of sticky class roots, pointing several times to the same instances. As we keep the list of roots in memory, this wasted a lot of memory and slowed down the sorting of gc roots which happens right before path finding. This change fixes this by deduplicating StickyClass GC roots based on their ids.

[pyricau]

shark.HprofRetainedHeapPerfTest > freeze retained memory when indexing leak_asynctask_m FAILED
    java.lang.AssertionError: 
    Expecting:
     <4890760>
    to be between:
     [7789999, 8609998]

Android M = API 23, so of course we're using less memory now.