Docs: add a known issue for Android L-M emulators

No change to logic, only docs. This updates the emulator docs to call
out a known issue with Android L-M emulator images. Unlike other OS
versions, all L-M emulator images are configured to expect the AOSP
WebView package.

Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ia48ab33dab246b89c98cfc512bd5e0007f760f85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2806654
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#869554}

docs/android_emulator.md

@@ -202,6 +202,13 @@ Images** tab in the Virtual Device Configuration wizard.
    * To ensure it's there: `adb -s emulator-5554 shell mount` (look for /sdcard)
    * Can often be fixed by editing `~/.android/avd/YOUR_DEVICE/config.ini`.
      * Look for `hw.sdCard=no` and set it to `yes`
+ * The "Google APIs" Android L and M emulator images are configured to expect
+   the "AOSP" WebView package (`com.android.webview`). This does not resemble
+   production devices with GMS, which expect the ["Google WebView"
+   configuration](/android_webview/docs/webview-providers.md#webview-provider-options)
+   (`com.google.android.webview` on L and M). See [Removing preinstalled
+   WebView](/android_webview/docs/build-instructions.md#Removing-preinstalled-WebView)
+   if you need to install a local build or official build.
 
 
 #### Starting an Emulator from the Command Line