decouple compileSdk version from build tools version (#21205)

Summary:
Android CI was failing due to that it was trying to extract major version from build tools and use it to compile ReactAndroid. Now, it'll extract compileSdkVersion from ReactAndroid/build.gradle and use it.

Issue was that 68c7999 dowgraded compileSdk version to 26 while retaining build tools version as 27.0.3, so CI was trying to use SDK 27.
Pull Request resolved: #21205

Differential Revision: D9943909

Pulled By: hramos

fbshipit-source-id: ec9bc0c40956a16f8088532340722fd43cadde37

scripts/validate-android-test-env.sh

@@ -56,7 +56,7 @@ fi
 BUILD_TOOLS_VERSION=`grep buildToolsVersion $(dirname $0)/../ReactAndroid/build.gradle | sed 's/^[^"]*\"//' | sed 's/"//'`
 
 # MAJOR is something like "23"
-MAJOR=`echo $BUILD_TOOLS_VERSION | sed 's/\..*//'`
+MAJOR=`grep compileSdkVersion $(dirname $0)/../ReactAndroid/build.gradle | sed 's/[^[:digit:]]//g'`
 
 # Check that we have the right major version of the Android SDK.
 PLATFORM_DIR="$ANDROID_HOME/platforms/android-$MAJOR"