[AppBar] Always apply the default shadow behavior when theming.

[jverkoey]

Both Primary and Surface theming are expected to have a shadow when scrolled. See https://material.io/components/app-bars-top/#theming for more details.

This fixes a bug where the AppBar's Primary theming implementation would set a shadow layer that would not react to scrolling.

Example before/after of a Surface-themed app bar (note the darker shadow in alignment with the expected behavior)

Before	After

Related to https://github.com/material-components/material-components-ios/issues/6332

[material-automation]

bazel detected changes to the following targets:

//components/AppBar:ObjcExamples
//components/AppBar:SwiftExamples
//components/AppBar:Theming
//components/AppBar:snapshot_test_lib
//components/AppBar:snapshot_tests
//components/AppBar:snapshot_tests.swift_runtime_linkopts
//components/AppBar:snapshot_tests_test_binary
//components/AppBar:snapshot_tests_test_bundle
//components/AppBar:unit_test_sources
//components/AppBar:unit_tests
//components/AppBar:unit_tests_IPAD_PRO_12_9_IN_9_3
//components/AppBar:unit_tests_IPAD_PRO_12_9_IN_9_3.swift_runtime_linkopts
//components/AppBar:unit_tests_IPAD_PRO_12_9_IN_9_3_test_binary
//components/AppBar:unit_tests_IPAD_PRO_12_9_IN_9_3_test_bundle
//components/AppBar:unit_tests_IPHONE_7_PLUS_IN_10_3
//components/AppBar:unit_tests_IPHONE_7_PLUS_IN_10_3.swift_runtime_linkopts
//components/AppBar:unit_tests_IPHONE_7_PLUS_IN_10_3_test_binary
//components/AppBar:unit_tests_IPHONE_7_PLUS_IN_10_3_test_bundle
//components/AppBar:unit_tests_environment
//components/AppBar:unit_tests_environment.swift_runtime_linkopts
//components/AppBar:unit_tests_environment_test_binary
//components/AppBar:unit_tests_environment_test_bundle
//components/Banner:ObjcExamples

[material-automation]

clang-format suggested the following change:

-  ^(CALayer *_Nonnull shadowLayer, CGFloat intensity) {
-    CGFloat elevation = MDCShadowElevationAppBar * intensity;
-    weakSelf.headerView.elevation = elevation;
-    [(MDCShadowLayer *)shadowLayer setElevation:elevation];
-  };
+      ^(CALayer *_Nonnull shadowLayer, CGFloat intensity) {
+        CGFloat elevation = MDCShadowElevationAppBar * intensity;
+        weakSelf.headerView.elevation = elevation;
+        [(MDCShadowLayer *)shadowLayer setElevation:elevation];
+      };