Remove SHA1 check from AndroidDevice.isAppInstalled() (#8290)

* Remove SHA1 check from AndroidDevice.isAppInstalled()

The docs for isAppInstalled say 'check if a version of the given app is
already installed', however the current code returns true only if it's
the latest build that's installed.

This made sense in the past, when the use pattern was:

  if (!isAppInstalled(...)) installApp(...);

but now the usage is:

  if (isAppInstalled(...)) uninstallApp(...);
  installApp(...);

This has the probably unintended consequence that if you run `flutter
install` or `flutter run` two times in a row with no source changes, the
second invocation will uninstall the app, but the first invocation might
not.

Removing the SHA1 check makes us always uninstall the app if it's
installed.

Fixes #8172