When getting SharedPreferences of other termux sharedUserId app packages, we get its Context first and if its null, it would mean that the package is not installed or likely has a different signature. For this case, we force exit the app in some places, since that shouldn't occur. Previously, if it was null, we were defaulting to getting SharedPreferences of current package context instead, which would mix keys of other packages with current one. SharedPreferences of other app packages aren't being used currently, so this isn't an issue, this commit just fixes the issue for future.
Force exit will also be triggered if Termux is forked and TermuxConstants.TERMUX_PACKAGE_NAME is not updated to the same value as applicationId since TermuxActivity.onCreate() will fail to get SharedPreferences of TermuxConstants.TERMUX_PACKAGE_NAME.
Moreover, its normally not allowed to install apps with different signatures, but if its done, we "may" need AndroidManifest `queries` entries in andorid 11, check PackageUtils.getSigningCertificateSHA256DigestForPackage() for details.
