Support R8 full mode for lich-viewmodel

[ganadist]

We found classes can be merged aggressively on R8 full mode.
This change will help to migrate R8 full mode on android apps which are relied on lich viewmodel.

Test

$ ./gradlew :viewmodel:assembleRelease

$ zipgrep keepnames viewmodel/build/outputs/aar/viewmodel-release.aar
proguard.txt:# -keepnames (= -keep,allowshrinking) only pins the class *name*: it prevents
proguard.txt:-keepnames class * extends com.linecorp.lich.viewmodel.ViewModelFactory

[CLAassistant]

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