Android: use AndroidX classes in styleguide

This updates the styleguide to recommend androidx.annotation.Nullable
because android.support.* is generally deprecated. We've already
migrated chromium to the AndroidX imports, so this just updates the
styleguide to be in line with our current practices.

This deletes the word "other" from the section header because this is
the only reference I could find to Java annotations.

Test: tools/md_browser/md_browser.py styleguide/java/java.md
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I84a78e3396fc05eeabda1114aab1658593a9be66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593182
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837588}

styleguide/java/java.md

@@ -124,12 +124,12 @@ Classes that need destructor logic should provide an explicit `destroy()`
 method. Use [LifetimeAssert](https://chromium.googlesource.com/chromium/src/+/master/base/android/java/src/org/chromium/base/LifetimeAssert.java)
 to ensure in debug builds and tests that `destroy()` is called.
 
-### Other Android Support Library Annotations
+### AndroidX Annotations
 * Use them! They are [documented here](https://developer.android.com/studio/write/annotations).
   * They generally improve readability.
   * Some make lint more useful.
-* `javax.annotation.Nullable` vs `android.support.annotation.Nullable`
-  * Always prefer `android.support.annotation.Nullable`.
+* `javax.annotation.Nullable` vs `androidx.annotation.Nullable`
+  * Always prefer `androidx.annotation.Nullable`.
   * It uses `@Retention(SOURCE)` rather than `@Retention(RUNTIME)`.
 
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