[mono] Unify mobile build integration files and sample apps for non-Mono runtimes

[ivanpovazan]

Description

Unify and cleanup mobile samples and build integration files for non-Mono runtimes.
The idea is to use TargetRuntime property with Mono, CoreCLR, NativeAOT values, instead of UseNativeAOTRuntime UseMonoRuntime, similarly to:

runtime/src/mono/msbuild/apple/build/AppleBuild.targets

Lines 303 to 305 in f1901a0

	<AppleAppBuilderRuntime Condition="'$(AppleAppBuilderRuntime)' == '' and '$(UseNativeAOTRuntime)' == 'true'">NativeAOT</AppleAppBuilderRuntime>
	<AppleAppBuilderRuntime Condition="'$(AppleAppBuilderRuntime)' == '' and '$(UseMonoRuntime)' == 'false'">CoreCLR</AppleAppBuilderRuntime>
	<AppleAppBuilderRuntime Condition="'$(AppleAppBuilderRuntime)' == ''">MonoVM</AppleAppBuilderRuntime>

This will also require us to clean-up internal usage of UseMonoRuntime and RuntimeFlavor properties (see: #111919)

TODOs:

• Address comment about different ways of importing runtime include files
• Address comment by adding a Error message for unsupported library mode configuration
• Address comment1 comment2 by sharing more code in hosts

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