Turn FakeObserver into a anonymous object

The lint seems to trip up parsing the inline class, so change it into an
anonymous object.

Bug: 439566448
Bug: 439453044
Test: re-enable lint and run build/soong/soong_ui.bash --make-mode -j64 TARGET_PRODUCT=mainline_modules_x86_64 DIST_DIR=out/dist/mainline_modules_x86_64 'TARGET_BUILD_APPS=DocumentsUIGoogle DocumentsUIGoogleTests ' ALWAYS_EMBED_NOTICES=true MODULE_BUILD_FROM_SOURCE=true EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true CLANG_COVERAGE=true CLANG_COVERAGE_CONTINUOUS_MODE=false ANDROID_LINT_CHECK=JavaKotlinApiUsedByModule ANDROID_LINT_CHECK_EXTRA_MODULES=JavaKotlinApiFinder SOONG_COLLECT_JAVA_DEPS=true APEX_BUILD_FOR_PRE_S_DEVICES=true out/host/linux-x86/bin/merge_zips lint-check module-info apps_only dist
Flag: EXEMPT test only
Change-Id: I00414f325e73d21e2a72128bfef5c4f124e5de26