[One .NET] specify all RIDs by default

Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1413756
Fixes: dotnet#6353

The API 31 emulator no longer has 32-bit images and the x86_64 image
has dropped support for 32-bit architectures:

    > adb shell getprop | grep cpu
    [ro.product.cpu.abi]: [x86_64]
    [ro.product.cpu.abilist]: [x86_64,arm64-v8a]
    [ro.product.cpu.abilist32]: []
    [ro.product.cpu.abilist64]: [x86_64,arm64-v8a]

Compared to an API 30 x86_64 emulator:

    > adb shell getprop | grep cpu
    [ro.product.cpu.abi]: [x86_64]
    [ro.product.cpu.abilist]: [x86_64,x86,arm64-v8a,armeabi-v7a,armeabi]
    [ro.product.cpu.abilist32]: [x86,armeabi-v7a,armeabi]
    [ro.product.cpu.abilist64]: [x86_64,arm64-v8a]

The problem is our default RIDs are:

    <RuntimeIdentifiers Condition=" '$(RuntimeIdentifier)' == '' And '$(RuntimeIdentifiers)' == '' ">android-arm64;android-x86</RuntimeIdentifiers>

And so you hit this error when trying to deploy a `dotnet new android`
app on an API 31 x86_64 emulator:

    error ADB0020: Mono.AndroidTools.IncompatibleCpuAbiExceptiopn: The package does not support the CPU architecture of this device.

To workaround this, you can add `android-x64` to your list of
`$(RuntimeIdentifiers)`.

To solve this issue, we can default `$(RuntimeIdentifiers)` to all 4
architectures. We have code that will select a single architecture for
Debug builds using "Fast Deployment". It seems better to have a
default here that will always work, and the only drawback would be the
additional architectures for Release builds.

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets

@@ -66,7 +66,7 @@
     <TrimmerDefaultAction Condition=" '$(TrimmerDefaultAction)' == '' and '$(AndroidLinkMode)' == 'Full' ">link</TrimmerDefaultAction>
     <SuppressTrimAnalysisWarnings Condition=" '$(SuppressTrimAnalysisWarnings)' == '' ">true</SuppressTrimAnalysisWarnings>
     <!-- Prefer $(RuntimeIdentifiers) plural -->
-    <RuntimeIdentifiers Condition=" '$(RuntimeIdentifier)' == '' And '$(RuntimeIdentifiers)' == '' ">android-arm64;android-x86</RuntimeIdentifiers>
+    <RuntimeIdentifiers Condition=" '$(RuntimeIdentifier)' == '' And '$(RuntimeIdentifiers)' == '' ">android-arm;android-arm64;android-x86;android-x64</RuntimeIdentifiers>
     <RuntimeIdentifier  Condition=" '$(RuntimeIdentifiers)' != '' And '$(RuntimeIdentifier)' != '' " />
     <AndroidManifest Condition=" '$(AndroidManifest)' == '' and Exists ('Properties\AndroidManifest.xml') and !Exists ('AndroidManifest.xml') ">Properties\AndroidManifest.xml</AndroidManifest>
     <AndroidManifest Condition=" '$(AndroidManifest)' == '' ">AndroidManifest.xml</AndroidManifest>