Simplify stub TextServicesManager in layoutlib

As a preparation CL to work on Bug 7254002, this CL simplifies
the layoutlib mock class of TextServicesManager, which was mostly
as-is copied from the real one by a previous CL [1].

Although keeping maintaining one real class and one stub class is
still painful, it should be much better than maintaining the real
one and mostly the real one.

 [1]: I8c25f4919a2e97b56f705b59747e81cb6f45b07b
      f149ca78

Test: make layoutlib-tests layoutlib
Test: No new test failure in the following command.
      java -Dtest_res.dir=frameworks/base/tools/layoutlib/bridge/tests/res -cp .:/usr/share/java/junit4.jar:prebuilts/tools/common/m2/repository/junit/junit/4.12/junit-4.12.jar:prebuilts/tools/common/m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:prebuilts/misc/common/tools-common/tools-common-prebuilt.jar:prebuilts/misc/common/sdk-common/sdk-common.jar:prebuilts/misc/common/layoutlib_api/layoutlib_api-prebuilt.jar:prebuilts/misc/common/kxml2/kxml2-2.3.0.jar:prebuilts/tools/common/m2/repository/com/google/guava/guava/15.0/guava-15.0.jar:out/host/linux-x86/framework/layoutlib-tests.jar:out/host/linux-x86/framework/layoutlib.jar org.junit.runner.JUnitCore com.android.layoutlib.bridge.intensive.Main
Bug: 7254002
Change-Id: I03e95bae15dc04cc0df20c4fb7cc86c5f3c6ccde