Utility to merge multiple Bundles.

As part of our work on the "modern" broadcast queue, we're likely
going to need to collapse multiple existing broadcasts together,
using various strategies based on the contents of the extras.

Intents at a high level already have Intent.fillIn() to describe how
to be blended together, and this new class describes how to blend a
Bundle of extras together.  Common examples are included in
testMerge_PackageChanged() and testMerge_DropBox().

We have a strong preference to have the merging strategy described
in a static way so that it can be applied without the risk of an
open-ended callback while a critical lock is being held.  This also
aids the ability for remote processes to influence how their
broadcasts are merged.

Bug: 249160234
Test: atest FrameworksCoreTests:BundleMergerTest
Change-Id: Ibe8623c02147afbb99826dccb063bd566d447642