Update ART tests to handle new System.gc behavior

Changed calls from System.gc to Runtime.getRuntime.gc where it was
necessary.

Required for:
https://android-review.googlesource.com/#/c/80253/

Change-Id: I2b0622585da54229a6248e95d40134b6d18598a9
Change thread.h to thread-inl.h to pick up missing Thread::Currnet for debug build in master

Change-Id: I56a4dd18ec1c212f9dbb73b14c0c0623b23c87bd

Conflicts:
	runtime/base/logging.cc