Creates a generic mechanism to dump app-side information.

Currently, 'dumpsys activity' can dump the state of some managers:
 - AutofillManager
 - ContentCapturemaanger
 - UiTranslationController

But the support for these custom dumping is hardcoded into
Activity itself, which makes it harder to extend. For example,
automotive builds provide an app-side Car object, which currently
cannot be dumped.

This CL makes the mechanism more flexible by providing a couple new
public / SystemAPIs that let Automotive (or other mainline modules)
extend it.

Examples:

$ adb shell dumpsys activity com.android.car.carlauncher/.CarLauncher --list-dumpables
$ adb shell dumpsys activity com.android.car.carlauncher/.CarLauncher --dump-dumpable CarUserManager
$ adb shell dumpsys activity service com.android.systemui/.SystemUIService CarUserManager

NOTE: this CL only adds the new APIs; a follow-up CL will change the
existing managers to use them.

Test: see above
Test: m update-api

Bug: 149254050
CTS-Coverage-Bug: 149254050

Change-Id: I6920ff3542d3d75edd667c2c7658e9d0a7af534f