Ensure line numbers are preserved in default Proguard config

Explicitly keep the `LineNumberTable,SourceFile` attributes, ensuring
positional information is preserved for stack traces. While this isn't
required in R8's Proguard compat mode (still the default in soong), it
is needed for R8's full mode or when optimizations are enabled.

Bug: 215530220
Test: m
Change-Id: If1994e92f694c1a82317d79267714ad8114f3d99