fix target sdk version variable in build files

Author: yigit

BasicRxJavaSample/app/build.gradle

@@ -7,7 +7,7 @@ android {
     defaultConfig {
         applicationId "com.example.android.persistence"
         minSdkVersion build_versions.min_sdk
-        targetSdkVersion build_versions.targetSdkVersion
+        targetSdkVersion build_versions.target_sdk
         versionCode 1
         versionName "1.0"
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

BasicRxJavaSampleKotlin/app/build.gradle

@@ -26,7 +26,7 @@ android {
     defaultConfig {
         applicationId "com.example.android.observability"
         minSdkVersion build_versions.min_sdk
-        targetSdkVersion build_versions.targetSdkVersion
+        targetSdkVersion build_versions.target_sdk
         versionCode 1
         versionName "1.0"
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

GithubBrowserSample/app/build.gradle

@@ -22,7 +22,7 @@ android {
     defaultConfig {
         applicationId "com.android.example.github"
         minSdkVersion build_versions.min_sdk
-        targetSdkVersion build_versions.targetSdkVersion
+        targetSdkVersion build_versions.target_sdk
         versionCode 1
         versionName "1.0"
         testInstrumentationRunner "com.android.example.github.util.GithubTestRunner"

PersistenceContentProviderSample/app/build.gradle

@@ -22,7 +22,7 @@ android {
     defaultConfig {
         applicationId "com.example.android.contentprovidersample"
         minSdkVersion build_versions.min_sdk
-        targetSdkVersion build_versions.targetSdkVersion
+        targetSdkVersion build_versions.target_sdk
         versionCode 1
         versionName "1.0"
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

PersistenceMigrationsSample/app/build.gradle

@@ -23,7 +23,7 @@ android {
     defaultConfig {
         applicationId "com.example.android.persistence.migration"
         minSdkVersion build_versions.min_sdk
-        targetSdkVersion build_versions.targetSdkVersion
+        targetSdkVersion build_versions.target_sdk
         versionCode 1
         versionName "1.0"
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"