Fix accidental IMT and root marking regression

Was always using the conflict trampoline. Also included fix for
regression in GC time caused by extra roots. Most of the regression
was IMT.

Fixed bug in DumpGcPerformanceInfo where we would get SIGABRT due to
detached thread.

From ~2500 -> ~1980
GC time: 8.2s -> 7.2s due to 1s less of MarkConcurrentRoots

Bug: 19264997
Change-Id: I4333e80a8268c2ed1284f87f25b9f113d4f2c7e0
6 files changed