Adjust NDK platform version for compatibility

NDK platform version should not be higher than target SDK version in
build.gradle.

Signed-off-by: Leo Ma <begeekmyfriend@gmail.com>

library/src/main/libenc/jni/Application.mk

 APP_ABI := armeabi-v7a arm64-v8a x86
-APP_PLATFORM := android-24
+APP_PLATFORM := android-21

library/src/main/libenc/jni/libx264/android_build_arm64_v8a.sh

 #!/bin/sh
 ANDROID_NDK=/home/leoma/MyOSP/android-ndk-r13b
-SYSROOT=$ANDROID_NDK/platforms/android-24/arch-arm64
+SYSROOT=$ANDROID_NDK/platforms/android-21/arch-arm64
 CROSS_PREFIX=$ANDROID_NDK/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/bin/aarch64-linux-android-
 EXTRA_CFLAGS="-march=armv8-a -D__ANDROID__"
 EXTRA_LDFLAGS="-nostdlib"

library/src/main/libenc/jni/libx264/android_build_armeabi_v7a.sh

 #!/bin/sh
 ANDROID_NDK=/home/leoma/MyOSP/android-ndk-r13b
-SYSROOT=$ANDROID_NDK/platforms/android-24/arch-arm
+SYSROOT=$ANDROID_NDK/platforms/android-21/arch-arm
 CROSS_PREFIX=$ANDROID_NDK/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-
 EXTRA_CFLAGS="-march=armv7-a -mfloat-abi=softfp -mfpu=neon -D__ANDROID__ -D__ARM_ARCH_7__ -D__ARM_ARCH_7A__"
 EXTRA_LDFLAGS="-nostdlib"

library/src/main/libenc/jni/libx264/android_build_x86.sh

 #!/bin/sh
 ANDROID_NDK=/home/leoma/MyOSP/android-ndk-r13b
-SYSROOT=$ANDROID_NDK/platforms/android-24/arch-x86
+SYSROOT=$ANDROID_NDK/platforms/android-21/arch-x86
 CROSS_PREFIX=$ANDROID_NDK/toolchains/x86-4.9/prebuilt/linux-x86_64/bin/i686-linux-android-
 EXTRA_CFLAGS="-D__ANDROID__ -D__i686__"
 EXTRA_LDFLAGS="-nostdlib"

library/src/main/libenc/jni/libyuv/jni/Application.mk

 APP_ABI := armeabi-v7a arm64-v8a x86
-APP_PLATFORM := android-24
+APP_PLATFORM := android-21