Assert Android API >= v21 (#27943)

* Assert Android API >= v21

This precedes a change that would otherwise break on older Android APIs,
but in a more obvious way.

* error if __ANDROID_API__ is not defined

* update all Andriod minSdkVersions to 21

* csharp experimental: android-19 to 21

examples/android/helloworld/app/build.gradle

@@ -4,7 +4,7 @@ android {
     compileSdkVersion 26
     defaultConfig {
         applicationId "io.grpc.android.cpp.helloworldexample"
-        minSdkVersion 14
+        minSdkVersion 21
         targetSdkVersion 26
         versionCode 1
         versionName "1.0"

examples/csharp/HelloworldUnity/ProjectSettings/ProjectSettings.asset

@@ -153,7 +153,7 @@ PlayerSettings:
     iOS: com.jattermusch.grpc.example
   buildNumber: {}
   AndroidBundleVersionCode: 1
-  AndroidMinSdkVersion: 16
+  AndroidMinSdkVersion: 21
   AndroidTargetSdkVersion: 0
   AndroidPreferredInstallLocation: 1
   aotOptions: 

include/grpc/impl/codegen/port_platform.h

@@ -126,11 +126,15 @@
 #elif defined(ANDROID) || defined(__ANDROID__)
 #define GPR_PLATFORM_STRING "android"
 #define GPR_ANDROID 1
-#ifdef __ANDROID_API__
+#ifndef __ANDROID_API__
+#error "__ANDROID_API__ must be defined for Android builds."
+#endif
+#if __ANDROID_API__ < 21
+#error "Requires Android API v21 and above"
+#endif
 #if (__ANDROID_API__) >= 23
 #define GPR_SUPPORT_BINDER_TRANSPORT 1
 #endif
-#endif
 // TODO(apolcyn): re-evaluate support for c-ares
 // on android after upgrading our c-ares dependency.
 // See https://github.com/grpc/grpc/issues/18038.

src/android/test/interop/app/build.gradle

@@ -4,7 +4,7 @@ android {
     compileSdkVersion 26
     defaultConfig {
         applicationId "io.grpc.android.interop.cpp"
-        minSdkVersion 14
+        minSdkVersion 21
         targetSdkVersion 26
         versionCode 1
         versionName "1.0"

src/csharp/experimental/build_native_ext_for_android.sh

@@ -36,7 +36,7 @@ cd build
 # ANDROID_ABI in ('arm64-v8a', 'armeabi-v7a')
 # e.g. ANDROID_ABI=armeabi-v7a
 
-# android-19 corresponds to Kitkat 4.4
+# android-21 corresponds to Lollipop 5.0
 ${ANDROID_SDK_CMAKE} ../.. \
   -DCMAKE_TOOLCHAIN_FILE="${ANDROID_NDK_PATH}/build/cmake/android.toolchain.cmake" \
   -DCMAKE_ANDROID_NDK="${ANDROID_NDK_PATH}" \
@@ -45,7 +45,7 @@ ${ANDROID_SDK_CMAKE} ../.. \
   -DRUN_HAVE_STD_REGEX=0 \
   -DRUN_HAVE_STEADY_CLOCK=0 \
   -DCMAKE_BUILD_TYPE=Release \
-  -DANDROID_PLATFORM=android-19 \
+  -DANDROID_PLATFORM=android-21 \
   -DANDROID_ABI="${ANDROID_ABI}" \
   -DANDROID_NDK="${ANDROID_NDK_PATH}" \
   -DgRPC_XDS_USER_AGENT_IS_CSHARP=ON