Add a proguard-rules-consumer rule to never strip appservices code

[jonalmeida]

When debugging https://github.com/mozilla-mobile/focus-android/issues/5879, we realized that all consumers of app services need to know to add these proguard rules to avoid unnecessary stripping of bytecode which the Android optimizer thinks is unused:

-keep class mozilla.appservices.** { *; }

Instead of requiring each app to know this, we could add it into the consumer-proguard-rules.pro so that clients of app services get this rule for free.

cc: @jhugman

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