Fix regex for proguard -check rules

There were two problems with these regexes:
1. it didn't support the syntax without curly braces:
  -checkdiscard @com.Foo class *
2. it ate newlines, which could break rules

Change-Id: I667ec65f2e9c8a06c812d62578e700792cf172a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498828
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820894}