App会通过下面的api来获取UserAgent：
String userAgent = telephonyManager.getMmsUserAgent();
//最后会调到 /packages/services/Telephony/src/com/android/phone/PhoneInterfaceManager.java里面获取config_mms_user_agent的值
/packages/services/Telephony/src/com/android/phone/PhoneInterfaceManager.java
7082 @Override
7083 public String getMmsUserAgent(int subId) {
7084 //TODO investigate if this API should require proper permission check in R b/133791609
7085 final long identity = Binder.clearCallingIdentity();
7086 try {
7087 return SubscriptionManager.getResourcesForSubId(getDefaultPhone().getContext(), subId)
7088 .getString(com.android.internal.R.string.config_mms_user_agent);
7089 } finally {
7090 Binder.restoreCallingIdentity(identity);
7091 }
7092 }
可以参考修改下面的两个路径下面的xml文件：
/device/mediatek/system/common/overlay/telephony/frameworks/base/core/res/res/values/config.xml
54 Android-Mms/0.1
55
56
57 http://www.google.com/oha/rdf/ua-profile-kila.xml
/device/mediatek/common/overlay/telephony/frameworks/base/core/res/res/values/config.xml
54 Android-Mms/0.1
55
56
57 http://www.google.com/oha/rdf/ua-profile-kila.xml
58
PS： 适用于Android Q版本 google messaging app