[Ravenwood] Always provide main thread

It is very difficult for test owners to know whether a main thread is
actually required. Besides, the Ravenwood environment itself relies on a
main thread.

Flag: EXEMPT host test change only
Bug: 292141694
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Change-Id: I88ed44f00ce6295dcb14619428a2c7a4bea0bbdb