Android: Use single quotes in proguard flag string literal

R8 doesn't parse the "" version, but is fine with ''.

Bug: 872904
Change-Id: I827798ac9b29031138397dcadde38567e3df63bb
Reviewed-on: https://chromium-review.googlesource.com/1176631Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583630}

base/android/proguard/enable_obfuscation.flags

@@ -5,4 +5,4 @@
 # As of August 11, 2016, obfuscation was found to save 660kb on our .dex size
 # and 53kb memory/process (through shrinking method/string counts).
 -renamesourcefileattribute PG
--repackageclasses ""
+-repackageclasses ''