[Cronet] Add proguard files for different Cronet packaging

components/cronet/android/proguard.cfg

--keep class org.chromium.net.impl.CronetUrlRequest$HeadersList
--keep class org.chromium.net.impl.ChromiumUrlRequest$ResponseHeadersMap
-
-# This constructor is called by the reflection from Cronet API. It cannot be
-# annotated with @org.chromium.base.annotations.AccessedByNative in order to
-# avoid the dependency on Chromium-Base Java classes.
--keep class org.chromium.net.impl.CronetEngineBuilderImpl {
-    public <init>(android.content.Context);
-}
-
-# Suppress unnecessary warnings.
--dontnote org.chromium.net.ProxyChangeListener$ProxyReceiver
--dontnote org.chromium.net.AndroidKeyStore
-# Needs 'void setTextAppearance(int)' (API level 23).
--dontwarn org.chromium.base.ApiCompatibilityUtils
-# Needs 'boolean onSearchRequested(android.view.SearchEvent)' (API level 23).
--dontwarn org.chromium.base.WindowCallbackWrapper
-
-# Generated for chrome apk and not included into cronet.
--dontwarn org.chromium.base.BuildConfig
--dontwarn org.chromium.base.library_loader.NativeLibraries
--dontwarn org.chromium.base.multidex.ChromiumMultiDexInstaller
-
-# Objects of this type are passed around by native code, but the class
-# is never used directly by native code. Since the class is not loaded, it does
-# not need to be preserved as an entry point.
--dontnote org.chromium.net.UrlRequest$ResponseHeadersMap
-# https://android.googlesource.com/platform/sdk/+/marshmallow-mr1-release/files/proguard-android.txt#54
--dontwarn android.support.**
-