[Xamarin.Android.Build.Tasks] improve incremental deploy in IDEs

[jonathanpeppers]

When doing some testing in the IDE, I noticed something odd for an
incremental deploy:

Building target "_BuildApkFastDev" completely.
Input file "obj\Debug\90\adb.props" is newer than output file "obj\Debug\90\android\bin\com.companyname.app14.apk".

This target took 2.470s in this case.

This was the Android App template in VS 2019 (not Xamarin.Forms).

I just modified a .cs file, how did adb.props change???

It turns out during Build, $(AdbTarget) is blank and during Install
it is properly set. That means that Install is currently always
running _BuildApkFastDev on incremental deployments.

I let the IDE team know, so they can see about sending $(AdbTarget)
during Build. However, they can't do this in every case, due to the
way the UI is setup. Depending on the user's selection, current
document, focus, etc. the play button in VS may not be enabled or even
know the current attached device.

This leads me to think we should really change something in our
MSBuild targets, which currently does:

<WriteLinesToFile
    Condition=" '$(DesignTimeBuild)' != 'True' "
    File="$(_AdbPropertiesCache)"
    Lines="AdbTarget=$(AdbTarget);AdbOptions=$(AdbOptions)"
    Overwrite="true"
    WriteOnlyWhenDifferent="true"
/>

If we didn't write this file when $(AdbTarget) is blank, the problem
would be avoided. However, we also should write the file if it does
not exist. If an Input to a target doesn't exist, it will run every
time. adb.props is used as Inputs for some targets.

I added a test for this scenario in the MSBuildDeviceIntegration
tests.