native_loader: Use ANDROID_ROOT environment variable

namespace android {

#if defined(__ANDROID__)

static constexpr const char* kPublicNativeLibrariesSystemConfig = "/system/etc/public.libraries.txt";

static constexpr const char* kPublicNativeLibrariesSystemConfigPathFromRoot = "/etc/public.libraries.txt";

static constexpr const char* kPublicNativeLibrariesVendorConfig = "/vendor/etc/public.libraries.txt";

class LibraryNamespaces {

  void Initialize() {

    std::vector<std::string> sonames;

    const char* android_root_env = getenv("ANDROID_ROOT");

    std::string root_dir = android_root_env != nullptr ? android_root_env : "/system";

    std::string public_native_libraries_system_config =

            root_dir + kPublicNativeLibrariesSystemConfigPathFromRoot;

    LOG_ALWAYS_FATAL_IF(!ReadConfig(kPublicNativeLibrariesSystemConfig, &sonames),

    LOG_ALWAYS_FATAL_IF(!ReadConfig(public_native_libraries_system_config, &sonames),

                        "Error reading public native library list from \"%s\": %s",

                        kPublicNativeLibrariesSystemConfig, strerror(errno));

                        public_native_libraries_system_config.c_str(), strerror(errno));

    // This file is optional, quietly ignore if the file does not exist.

    ReadConfig(kPublicNativeLibrariesVendorConfig, &sonames);