Start accepting Executors instead of Handlers.

After discussion in API council, our new best-practices are to have
developers provide an Executor to dispatch callbacks/listeners on,
instead of the previous guidance of using a Handler.

Define Context.getMainExecutor() to easily obtain an Executor
associated with the main thread of application.  This allows new
APIs to require a @NonNull Executor.  Also define a new
@CallbackExecutor auto-doc annotation that explains background and
points developers at new Context method above.

Test: cts-tradefed run commandAndExit cts-dev -m CtsContentTestCases -t android.content.cts.ContextTest#testMainLooper
Test: cts-tradefed run commandAndExit cts-dev -m CtsContentTestCases -t android.content.cts.ContextTest#testMainExecutor
Bug: 70348426
Change-Id: I536892bcd80fcfba1bb1ddf67648c08a26d7ddd2