Android: Improve lint suppression regex

Explicitly match only on literal . characters instead of any characters.
Add explanatory comment.

Bug: 1078229
Change-Id: I1f248d270ebd22f17aebaad55750d5bed9a275a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519897
Auto-Submit: Peter Wen <wnwen@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824128}

chrome/android/expectations/lint-suppressions.xml

@@ -13,8 +13,9 @@ Still reading?
   local as possible, i.e. by warning message or by file.
 - When adding new issues, please keep the issue ids in sorted order.
 -->
+  <!-- Ignore all lint errors in clank code. -->
   <issue id="all">
-    <ignore regexp="^../../clank/"/>
+    <ignore regexp="^\.\./\.\./clank/"/>
     <ignore regexp="^gen/clank/"/>
     <!-- obj can be a subdirectory under gen. -->
     <ignore regexp="/obj/clank/"/>