[android-toolchain] Install Android platform-tools r28 (dotnet#1858)

If you are running Android O or above as the Android target to test
`make run-apk-tests`, it will fail because of an `adb` server version
mismatch (39 vs. 40).  This is a common `adb` issue e.g.:

https://sqa.stackexchange.com/questions/20524/how-to-resolve-adb-server-version-32-doesnt-match-this-client-36-killin

Bump to Android platform-tools r28 so that we have an updated `adb`,
which will avoid the version mismatch error.

build-tools/android-toolchain/android-toolchain.projitems

@@ -15,7 +15,7 @@
       <HostOS>Linux</HostOS>
       <DestDir>build-tools\$(XABuildToolsFolder)</DestDir>
     </AndroidSdkItem>
-    <AndroidSdkItem Include="platform-tools_r27.0.1-linux">
+    <AndroidSdkItem Include="platform-tools_r28.0.0-linux">
       <HostOS>Linux</HostOS>
       <DestDir>platform-tools</DestDir>
     </AndroidSdkItem>
@@ -34,7 +34,7 @@
       <HostOS>Darwin</HostOS>
       <DestDir>build-tools\$(XABuildToolsFolder)</DestDir>
     </AndroidSdkItem>
-    <AndroidSdkItem Include="platform-tools_r27.0.1-darwin">
+    <AndroidSdkItem Include="platform-tools_r28.0.0-darwin">
       <HostOS>Darwin</HostOS>
       <DestDir>platform-tools</DestDir>
     </AndroidSdkItem>
@@ -53,7 +53,7 @@
       <HostOS>Windows</HostOS>
       <DestDir>build-tools\$(XABuildToolsFolder)</DestDir>
     </AndroidSdkItem>
-    <AndroidSdkItem Include="platform-tools_r27.0.1-windows">
+    <AndroidSdkItem Include="platform-tools_r28.0.0-windows">
       <HostOS>Windows</HostOS>
       <DestDir>platform-tools</DestDir>
     </AndroidSdkItem>