The Android docs provide some guideance around toasts, notifications and dialogs.
The main difference between toasts and dialogs is that toasts provide feedback unobtrusively whereas dialogs demand an immediate response.
A toast provides simple feedback about an operation in a small popup.
It only fills the amount of space required for the message and the
current activity remains visible and interactive. For example,
navigating away from an email before you send it triggers a "Draft
saved" toast to let you know that you can continue editing later.
Toasts automatically disappear after a timeout.
A toast would not be appropriate if the user is expected to respond because it only appears briefly and cannot be recalled. In this situation, the Android docs recommend using a Notification.
If user response to a status message is required, consider instead
using a Notification.
Alerts in modal dialogs should be used whenever an explicit response is needed from the user, and that response is needed before anything else can happen.
A dialog does not fill the screen and is normally used for modal
events that require users to take an action before they can proceed.