pw_bluetooth_sapphire: Improve naming of android namespace aliases

Many files currently have the following at the top:

```
namespace hci_android = hci_spec::vendor::android;
namespace android_hci = pw::bluetooth::vendor::android_hci;
```

With two namespace aliases that look very similar, it's quite easy to
get confused. This change updates the above namespace aliases and their
usage to the following:

```
namespace android_hci = hci_spec::vendor::android;
namespace android_emb = pw::bluetooth::vendor::android_hci;
```

Fixed: b/335491380
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1034693
GitOrigin-RevId: bbf4261aa4090212ae02f417655ec78e13f2e421
Change-Id: I6cb98312bb9cf872981a85ab7ac5eb582e65c3b2
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/221232
Pigweed-Auto-Submit: Faraaz Sareshwala <fsareshwala@google.com>
Reviewed-by: Faraaz Sareshwala <fsareshwala@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Jason Graffius <jgraff@google.com>