Fix use of ANDROID_API environment variable in Android plugin

Author: thunderbiscuit

bdk-android/plugins/src/main/kotlin/org/bitcoindevkit/plugins/UniFfiAndroidPlugin.kt

@@ -36,7 +36,7 @@ internal class UniFfiAndroidPlugin : Plugin<Project> {
             environment(
                 // add build toolchain to PATH
                 Pair("PATH", "${System.getenv("PATH")}:${System.getenv("ANDROID_NDK_ROOT")}/toolchains/llvm/prebuilt/$llvmArchPath/bin"),
-                Pair("CFLAGS", "-D__ANDROID_API__=21"),
+                Pair("CFLAGS", "-D__ANDROID_MIN_SDK_VERSION__=21"),
                 Pair("CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER", "aarch64-linux-android21-clang"),
                 Pair("CC", "aarch64-linux-android21-clang")
             )
@@ -65,7 +65,7 @@ internal class UniFfiAndroidPlugin : Plugin<Project> {
             environment(
                 // add build toolchain to PATH
                 Pair("PATH", "${System.getenv("PATH")}:${System.getenv("ANDROID_NDK_ROOT")}/toolchains/llvm/prebuilt/$llvmArchPath/bin"),
-                Pair("CFLAGS", "-D__ANDROID_API__=21"),
+                Pair("CFLAGS", "-D__ANDROID_MIN_SDK_VERSION__=21"),
                 Pair("CARGO_TARGET_X86_64_LINUX_ANDROID_LINKER", "x86_64-linux-android21-clang"),
                 Pair("CC", "x86_64-linux-android21-clang")
             )
@@ -94,7 +94,7 @@ internal class UniFfiAndroidPlugin : Plugin<Project> {
             environment(
                 // add build toolchain to PATH
                 Pair("PATH", "${System.getenv("PATH")}:${System.getenv("ANDROID_NDK_ROOT")}/toolchains/llvm/prebuilt/$llvmArchPath/bin"),
-                Pair("CFLAGS", "-D__ANDROID_API__=21"),
+                Pair("CFLAGS", "-D__ANDROID_MIN_SDK_VERSION__=21"),
                 Pair("CARGO_TARGET_ARMV7_LINUX_ANDROIDEABI_LINKER", "armv7a-linux-androideabi21-clang"),
                 Pair("CC", "armv7a-linux-androideabi21-clang")
             )