tp: rename SparseVector -> NullableVector

Given that the container can be both dense and sparse, it makes sense to
rename to better reflect its true purpose.

Change-Id: I4b558cf46819a34559e489c293bc4bc7eacd0368
diff --git a/Android.bp b/Android.bp
index dfb1249..886f6a3 100644
--- a/Android.bp
+++ b/Android.bp
@@ -6481,8 +6481,8 @@
   srcs: [
     "src/trace_processor/containers/bit_vector.cc",
     "src/trace_processor/containers/bit_vector_iterators.cc",
+    "src/trace_processor/containers/nullable_vector.cc",
     "src/trace_processor/containers/row_map.cc",
-    "src/trace_processor/containers/sparse_vector.cc",
     "src/trace_processor/containers/string_pool.cc",
   ],
 }
@@ -6493,8 +6493,8 @@
   srcs: [
     "src/trace_processor/containers/bit_vector_unittest.cc",
     "src/trace_processor/containers/null_term_string_view_unittest.cc",
+    "src/trace_processor/containers/nullable_vector_unittest.cc",
     "src/trace_processor/containers/row_map_unittest.cc",
-    "src/trace_processor/containers/sparse_vector_unittest.cc",
     "src/trace_processor/containers/string_pool_unittest.cc",
   ],
 }