[Mono.Android] Bind API-37.2 Beta 3 - #12421

The *existence* of an Android QPR1 API-37.1 release alongside an
Android QPR2 API-37.2 beta release throws us into a new scenario: the
need for (at least?) *three* workload packs:

  * API-37 [stable]
  * API-37.1 [currently unstable, but see dotnet#12418]
  * API-37.2 [unstable]

(It would not surprise @jonpryor if there's a 4th by the time we're
done with API-37.*…)

However, thus far we've only ever expected *two* packages, so the
need for a third is unique.

Add a new `@(AndroidBuildApiLevel)` item group to
`Configuration.props`, which lists all the Android API levels to
build, along with the information required to build them (associated
framework version, platform id, is it *unstable*).

Update `ConfigureWorkload.targets` so that instead of "hardcoding"
`@(_FrameworkListOutputs)` against a specific list of of API levels
(`$(AndroidDefaultTargetDotnetApiLevel)` {?!},
`$(AndroidLatestStableApiLevel)`, `$(AndroidLatestUnstableApiLevel)`),
it uses `@(AndroidBuildApiLevel)`.  However, MSBuild item expansion
*does not like* using multiple item groups, e.g.:

	<_RuntimeListOutputs Include="…%(AndroidAbiAndRuntimeFlavor.AndroidRuntime).%(AndroidBuildApiLevel.Identity)…" />

Work around this problem by using a "cross join" across item groups
by using an intermediate Target + `@(_AndroidBuildInfo)` item group,
allowing use of a *single* item group:

	<_RuntimeListOutputs Include="…%(_AndroidBuildInfo.AndroidRuntime).%(_AndroidBuildInfo.Identity)…" />

Update the `BuildExtraApiLevels` target to build all API levels within
`@(AndroidBuildApiLevel)`, overriding MSBuild properties based on the
`@(AndroidBuildApiLevel)` item metadata.

Update `tests` so that the `_AddMonoAndroidReference` target finds
and uses *stable* bindings.  If it references an *unstable* binding,
we get a bunch of CS0246 errors as *nothing can be found*, as
`-p:EnablePreviewFeatures=true` is required when referencing an
unstable binding…