Use ScopedArenaAllocator for register allocation.

Memory needed to compile the two most expensive methods for
aosp_angler-userdebug boot image:
  BatteryStats.dumpCheckinLocked() : 25.1MiB -> 21.1MiB
  BatteryStats.dumpLocked(): 49.6MiB -> 42.0MiB
This is because all the memory previously used by Scheduler
is reused by the register allocator; the register allocator
has a higher peak usage of the ArenaStack.

And continue the "arena"->"allocator" renaming.

Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 64312607
Change-Id: Idfd79a9901552b5147ec0bf591cb38120de86b01
diff --git a/runtime/base/arena_object.h b/runtime/base/arena_object.h
index 2d8e7d8..ed00bab 100644
--- a/runtime/base/arena_object.h
+++ b/runtime/base/arena_object.h
@@ -32,8 +32,8 @@
     return allocator->Alloc(size, kAllocKind);
   }
 
-  static void* operator new(size_t size, ScopedArenaAllocator* arena) {
-    return arena->Alloc(size, kAllocKind);
+  static void* operator new(size_t size, ScopedArenaAllocator* allocator) {
+    return allocator->Alloc(size, kAllocKind);
   }
 
   void operator delete(void*, size_t) {
@@ -56,8 +56,8 @@
     return allocator->Alloc(size, kAllocKind);
   }
 
-  static void* operator new(size_t size, ScopedArenaAllocator* arena) {
-    return arena->Alloc(size, kAllocKind);
+  static void* operator new(size_t size, ScopedArenaAllocator* allocator) {
+    return allocator->Alloc(size, kAllocKind);
   }
 
   void operator delete(void*, size_t) {