land androidx in gradle (#24014)

Summary:
Use AndroidX in ReactAndroid/build.gradle, and remove androidx dependency from template and RNTester app because it's already exposed/exported from ReactAndroid.

[Android] [Changed] - Land AndroidX in gradle
Pull Request resolved: https://github.com/facebook/react-native/pull/24014

Differential Revision: D14508774

Pulled By: mdvacca

fbshipit-source-id: c96b97876571a5a7f2b400dd29188cfdf1f84a4c

RNTester/android/app/build.gradle

@@ -136,7 +136,6 @@ android {
 
 dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
-    implementation 'com.android.support:appcompat-v7:28.0.0'
 
     // Build React Native from source
     implementation project(':ReactAndroid')

ReactAndroid/build.gradle

@@ -319,7 +319,7 @@ android {
 dependencies {
     api("com.facebook.infer.annotation:infer-annotation:0.11.2")
     api("javax.inject:javax.inject:1")
-    api("com.android.support:appcompat-v7:28.0.0")
+    api("androidx.appcompat:appcompat:1.0.2")
     api("com.facebook.fresco:fresco:${FRESCO_VERSION}")
     api("com.facebook.fresco:imagepipeline-okhttp3:${FRESCO_VERSION}")
     api("com.facebook.soloader:soloader:${SO_LOADER_VERSION}")

template/android/app/build.gradle

@@ -153,7 +153,6 @@ android {
 
 dependencies {
     implementation fileTree(dir: "libs", include: ["*.jar"])
-    implementation "com.android.support:appcompat-v7:${rootProject.ext.supportLibVersion}"
     implementation "com.facebook.react:react-native:+"  // From node_modules
 }