AW docs: update Android branch to "main"

Android swapped from "master" to "main" for its branch name. This
updates references to point to the latest AOSP main branch.

I also made a small wording change to make the grammar flow more
naturally for one sentence.

Test: Upload to gerrit > open file > click "preview"
Change-Id: I9fb38ea5f2fbd5a2155fcedd1e2908c015775a83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4684341
Commit-Queue: Adam Walls <avvall@google.com>
Reviewed-by: Adam Walls <avvall@google.com>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1170784}

android_webview/docs/device-setup.md

@@ -50,7 +50,7 @@ Googlers can consult internal instructions
 [here](http://go/clank-webview/device_setup.md).
 
 External contributors can flash a prebuilt `userdebug` image (based off
-aosp-master) onto a Pixel device with [Android Flash
+aosp-main) onto a Pixel device with [Android Flash
 Tool](https://flash.android.com/welcome?continue=%2Fcustom). This requires a
 browser capable of WebUSB (we recommend the latest Google Chrome stable
 release).

android_webview/docs/threading.md

@@ -66,8 +66,8 @@ are not held off include:
 
 Views in android are expected to be created and used on its single **view**
 thread, unless otherwise noted in the API. For WebView, this is enforced for
-apps targeting JB MR2 or above. The code lives in the
-[WebView](https://cs.android.com/android/platform/superproject/+/master:frameworks/base/core/java/android/webkit/WebView.java).
+apps targeting JB MR2 or above. The code lives in
+[WebView.java](https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/core/java/android/webkit/WebView.java).
 Note this is an orthogonal concern to the single **UI** thread described above,
 as this check still allows different WebViews to be used on different **view**
 threads.