[Hotword] Add new flag for flipping hardware features for testing.

Updates v2 (experimental) flag description.
Also updates instances where always on should only be available if both flags are enabled.

BUG=432995

Review URL: https://codereview.chromium.org/724223002

Cr-Commit-Position: refs/heads/master@{#304666}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index cb0815a..1ad51bf 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1956,6 +1956,13 @@
     kOsDesktop,
     SINGLE_VALUE_TYPE(switches::kEnableExperimentalHotwording)
   },
+  {
+    "enable-hotword-hardware",
+    IDS_FLAGS_ENABLE_EXPERIMENTAL_HOTWORD_HARDWARE_NAME,
+    IDS_FLAGS_ENABLE_EXPERIMENTAL_HOTWORD_HARDWARE_DESCRIPTION,
+    kOsCrOS,
+    SINGLE_VALUE_TYPE(switches::kEnableExperimentalHotwordHardware)
+  },
 #if defined(ENABLE_EXTENSIONS)
   {
     "enable-embedded-extension-options",