[mono-runtimes] Use API-9 includes directory for 32-bit ARM (#909)

NDK 14+ (which we use) doesn't have android-4 anymore.

Author: grendello

build-tools/mono-runtimes/mono-runtimes.props

@@ -113,7 +113,7 @@
 
   <!-- Mono runtimes settings -->
   <PropertyGroup>
-    <_ArmNdkPlatformPath>$(AndroidToolchainDirectory)\ndk\platforms\android-4</_ArmNdkPlatformPath>
+    <_ArmNdkPlatformPath>$(AndroidToolchainDirectory)\ndk\platforms\android-9</_ArmNdkPlatformPath>
     <_ArmAr>$(AndroidToolchainDirectory)\toolchains\arm-linux-androideabi-clang\bin\arm-linux-androideabi-ar</_ArmAr>
     <_ArmAs>$(AndroidToolchainDirectory)\toolchains\arm-linux-androideabi-clang\bin\arm-linux-androideabi-as</_ArmAs>
     <_ArmCc>$(AndroidToolchainDirectory)\toolchains\arm-linux-androideabi-clang\bin\arm-linux-androideabi-clang</_ArmCc>