[Cards] Add umbrella headers for all extension targets. (#3463)

Also updated all imports to make use of the umbrella headers.

Pivotal story: https://www.pivotaltracker.com/story/show/157045283

components/Cards/src/ColorThemer/MaterialCards+ColorThemer.h

@@ -0,0 +1,17 @@
+/*
+ Copyright 2018-present the Material Components for iOS authors. All Rights Reserved.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+#import "MDCCardsColorThemer.h"

components/Cards/tests/unit/MDCCardColorThemerTests.swift

@@ -16,7 +16,7 @@
 
 import XCTest
 import MaterialComponents.MaterialCards
-import MaterialComponents.MDCCardsColorThemer
+import MaterialComponents.MaterialCards_ColorThemer
 
 class CardColorThemerTests: XCTestCase {