How can I get deep sleep -> coma for max battery life as comms platform? I'm not an Androidite, just very curious.
Arduinos have a timer-driven deep sleep mode that reduces their power requirements by 1000 or so.  But it's a specialized and limited device, and a building block rather than a system/platform.  You can add wifi or bluetooth to an arduino project - a good solution if you're near wifi or BT.
Is there any deep sleep thing like this for Android phones?  Something that will get weeks out of the phone battery, because it's NOT being used for "any time" communications?  I know I'm talking opposite for what the phone is designed for:  Always ON, always active, always waiting for in-bound calls, or responding to system events or user input.  Services STAY ON, "just in case".
I'm thinking that a way around the communication distance limitation for arduino would be to:  Use the arduino for sensing/deep sleep cycles, and tie it to a dedicated pre-paid phone for access to the data plan.
By use case / design, the phone isn't going to have any (many) in-bound calls - it will be used periodically as a hot-spot to allow the other device (arduino) to upload data to the web.  When the cheap, pre-paid device isn't actively sending data, I would expect it to go into a medically-induced coma until it's needed next.
A regular Android, in "standby" mode will last what... a couple of days?  I would want a way to make that battery to last weeks or more.
Without moding the phone to bypass the ON switch, is there a way change settings or whatever to totally hybernate/freeze/halt/drop/suspend most regular applications/etc to stretch out the available power as long as possible?  Either have the phone go comotose but with a schedule "wake-up and check"s to signal the arduino that it can transmit data; or have the arduino kick the phone into activity long enough to upload any data to the network.

An example use case for this would be:  I want to monitor nest activity, and water, air, soil temperature on an island in the middle of a lake.  I don't want to have to row out to the island to see the data. I don't have "real" telemetry gear / transceivers, etc.  I want my nifty little arduino to log the inputs and periodically, send a batch of data out to the cloud.  I want to use the existing telcom infrastructure via a cheap pre-paid cell phone, if I can, but I need it to be in service for weeks, so I want the phone do be so low power and so optimized that it only turns on to send a batch of data and then immediately goes dark again, until the next cycle.
I think you're looking for a feature called "Doze mode" in android. Its been around since Android Marshmallow (6.0.1) . If the phone is idle the device enters in a sleep mode, pretty much what you described in your first paragraph. It suspends all the background activity and saves battery for a long time. Doze mode is enabled by default in devices running Marshmallow or higher 

 Also you can use an app called  Greenify to hibernate other third party apps that you're not using. This also saves a lot of battery for me.hope this helps

A brief description about Doze mode in android