Fix thread suspension assertion in debugger.

Updates Dbg::GetThreadGroup to ensure we do call EndAssertNoThreadSuspension
after StartAssertNoThreadSuspension and not returning in the middle. Note this
only happens in debug mode where this assertion is enabled.

Also add missing thread safety annotations.

Bug: 13425576
Change-Id: Idb9f32d289038b77771369c1283774676ff433c7
2 files changed