Refinement of ContextParams behavior.

Since this parameters object is likely going to grow over time, we
should offer a way for callers to easily "inherit" all sub-parameters
by default, instead of making them copy the values manually.

This also gives a path for legacy apps to automatically preserve any
new sub-parameters that have been added in more modern API levels.

This change also relaxes the nullability annotations to match the
existing createAttributionContext() behavior, while also supporting
clearing of specific inherited values.

Bug: 181812281
Test: atest CtsContentTestCases:android.content.cts.ContextTest
Change-Id: Ic797e1aef578926b04ea0000ca3a1bf8684183b9