Move minSdkVersion default into AndroidX plugin

This changes the beahvior to set the minimum to the default (currently 14) and allow overriding using the normal mechanism of the android.defaultConfig { } DSL.

Library with default:

    $ cat ../../out/host/gradle/frameworks/support/core/build/intermediates/merged_manifests/release/processReleaseManifest/merged/AndroidManifest.xml | \grep minSdk
        <uses-sdk android:minSdkVersion="14" />

Library with custom value:

    $ cat ../../out/host/gradle/frameworks/support/heifwriter/build/intermediates/merged_manifests/release/processReleaseManifest/merged/AndroidManifest.xml | \grep minSdk
        <uses-sdk android:minSdkVersion="28" />

Test: ./gradlew :core:assemble :heifwriter:assemble (and manual verification above)
Change-Id: I5dfac73f480b49b697aff4bc3243b845e1449f98
diff --git a/docs-fake/build.gradle b/docs-fake/build.gradle
index ea70dfa..9c9dd5a 100644
--- a/docs-fake/build.gradle
+++ b/docs-fake/build.gradle
@@ -34,6 +34,10 @@
 }
 
 android {
+    defaultConfig {
+        minSdkVersion SupportConfig.CURRENT_SDK_VERSION
+    }
+
     sourceSets {
         main {
             manifest.srcFile "AndroidManifest.xml"
@@ -41,8 +45,3 @@
     }
     flavorDimensions("library-group")
 }
-
-
-supportTestApp {
-    minSdkVersion = SupportConfig.CURRENT_SDK_VERSION
-}
\ No newline at end of file