build: fixed building configuration for Android

Fixed architecture configuration and modified minimum Android SDK version.

In newer versions of the Android NDK, if the Android API version is lower than 24 (Android 7.0), compilation will not work because of API changes (in fact, Android API versions of 24 and below are no longer common), and the Android device architecture now only exists for ‘armv7a’, not ‘armv7’, which can also lead to compilation errors.

Reference: https://developer.android.com/studio/releases/platforms https://developer.android.com/ndk/guides/abis