Disable kotlin -checkdiscard rule

kotlin-stdlib is included transitively as a library jar by way of
androidx.annotations, which means we cannot ensure associated classes
are discarded by R8. Disable the related `-checkdiscard` proguard rule
until that inclusion is resolved, avoiding spurious warnings in build
output.

Bug: 302383328
Test: m
Flag: NA
Change-Id: I7514e6841735e95e1a16d42d6a6e134103ecb108