Optimize InputConnectionProtoDumper a bit

This is a mechanical refactoring CL that has no behavior change.

Currently all the utility methods defined in
InputConnectionProtoDumper return ProtoOutputStream, while the
returned instances will always be converted into byte[] eventually.
With this CL, those utility methods return byte[] instances directly,
which is expected to make it easier for ART/dexpreopt to do more
optimizations such as code inlining because instances of
ProtoOutputStream will no longer be escaped from those methods.

Bug: 192412909
Test: atest CtsInputMethodTestCases
Test: Manually verified that IME tracing still works
Change-Id: I7b24aee5428da312972aa86b8658429b421490f8