If your are looking for the value defined by applicationId in gradle, you can simply use
BuildConfig.APPLICATION_ID
EDIT 2022:
As indicated in the comments, BuildConfig.APPLICATION_ID is deprecated, but for librairies only. In this case, BuildConfig.LIBRARY_PACKAGE_NAME should be used instead, as explained here.
Also, now, Context::getPackageName returns the value defined by applicationId in gradle, even if you have several flavors with different applicationId (with a unique namespace), see here.