Split DumpManager into two

Splits DumpManager into two pieces:
[1] DumpHandler, which is responsible for parsing input args from either
    of our dumpable services and triggering the right kind of dumps.
[2] DumpManager, which is now largely just a registry of dumpables that
    exposes methods for dumping those dumpables.

This change is necessary to allow other clients to use the DumpManager
to dump to things other than bug reports.

Test: manual, atest
Bug: 112656837, 151317347
Change-Id: I6dcef9773c204c50fb4101f8fc355767b54f39bc