Password and confirm password validation in Android Studio
Inconsistent UX

Most validation errors call setError of the relevant input field.
An exception is when the confirmation password doesn't match the first entry,
that pops up a toast.
It would be better to handle validation errors consistently,
by using setError for mismatched confirmation password.

It's ok to use a toast for the result of the registration,
when all form fields are valid.

Creating account with invalid input

Before calling mAuth.createUserWithEmailAndPassword,
the only validation that's really enforced is that the confirmation password is not empty.
That's strange.
It would be better to call this only when all entries of the form are filled and valid.

Style issues

A convention in Java is to use camelCase for variable names.
For example RegisterConfirmPassword should be registerConfirmPassword.

Instead of if (a == false) { it's more natural to write if (!a) {.

It's a convention in Java to place braces like this:

if (cond) {
    // ...
} else {
    // ...
}