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Source: Android Developer

Question: How to prevent shrinkResources from deleting some files?

Answer:

Resouces shrinking is a way to help keep the szie of the apk down, for example:

android {
    ...
    buildTypes {
        release {
            shrinkResources true
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
                    'proguard-rules.pro'
        }
    }
}

This attribute must be set together with the minifyEnabled true attribute, see the link for detail.

But in case we need to keep some specific files:

If there are specific resources you wish to keep or discard, create an XML file in your project with a <resources> tag and specify each resource to keep in the tools:keep attribute and each resource to discard in the tools:discard attribute. Both attributes accept a comma-separated list of resource names. You can use the asterisk character as a wild card.

For example:

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="@layout/l_used*_c,@layout/l_used_a,@layout/l_used_b*"
    tools:discard="@layout/unused2" />

Save this file in your project resources, for example, at res/raw/keep.xml. The build does not package this file into your APK.