Bump Gradle minSdkVersion (#91)

Summary:
Pull Request resolved: #91

bd94aae bumped to NDK 26 (latest LTS NDK).

NDK 26 [dropped](https://github.com/android/ndk/wiki/Changelog-r26) [support](android/ndk#1751) for KitKat, and NDK 25 already required minSDK 19, so our minSdk of 15 is too low.

This bumps the minSdk, so that when we publish AARs for next fbjni published version, it errors if user is on earlier SDK.

Reviewed By: rshest

Differential Revision: D51146044

fbshipit-source-id: de1b38de0f2bd0afe9ed7ac074eaf7eb07903d40

build.gradle

@@ -36,7 +36,7 @@ android {
     }
 
     defaultConfig {
-        minSdkVersion 15
+        minSdkVersion 21
         targetSdkVersion 34
         buildConfigField "boolean", "IS_INTERNAL_BUILD", 'true'
         namespace "com.facebook.fbjni"