Provide proguard configuration for android artifacts

[mirceanis]

@mirceanis commented on Thu Jul 19 2018

When integrators use the signer library in an app that uses proguard, they should not have to struggle to find a working proguard configuration for the classes created here, nor one to patch the warnings.

This should be done by providing a consumerProguardFiles entry in signer/build.gradle

Apps may encounter warnings about:
javax.naming.** - because of some unused classes from spongycastle
java.lang.management.** - ??
org.slf4j.** - because of dependencies of kethereum
android.** - ??

definition of done

• signer demoapp has a release version with minification enabled
• signer demoapp does NOT need special proguard configuration to use the signer library. It should automatically propagate the proguard config