emulator: Set the property 'hw.keyboard' to "yes" before creating AVD.

When this property has the default value "no", AVDs with android-10
and later have issue with "back" and "home" buttons under graphical mode.
These buttons will not work as they should when clicked.

Setting the prop to "yes" will fix this issue.

Bug: 922145
Change-Id: I67322347f323d9b31927589b00624a41f5acc356
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2486288
Commit-Queue: Haiyang Pan <hypan@google.com>
Auto-Submit: Haiyang Pan <hypan@google.com>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819205}

build/android/pylib/local/emulator/avd.py

@@ -258,6 +258,7 @@ class AvdConfig(object):
 
         config_ini_contents.update({
             'disk.dataPartition.size': '4G',
+            'hw.keyboard': 'yes',
             'hw.lcd.density': density,
             'hw.lcd.height': height,
             'hw.lcd.width': width,