ART: Mark Dbg GCs as debugger

Change Heap::CollectGarbage to accept explicit GcCause, but implicitly
default to kGcCauseExplicit.

Change Dbg functions that run an explicit GC to set the cause to
kGcCauseDebugger.

Test: m test-art-host
Change-Id: I53d4073fca01c1de78d14a58dff33004c7971981