Stop supporting SerialManager.

Just was just as an example of using a system service, and now we
already use another system service (PlatformCompat).

On top of that, I heard a discussion about stopping support of this.

So let's just delete it.

Bug: 292141694
Flag: TEST_ONLY
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s
Change-Id: I89987a875cc8a78a43638c506d34ce896b372f11