Add "Android ashmem" page

Content provided by Alice Ryhl.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

SUMMARY.md

@@ -39,6 +39,7 @@
 ## Users — out-of-tree
 
   - [Android Binder Driver](Android-Binder-Driver.md)
+  - [Android `ashmem`](Android-`ashmem`.md)
   - [Apple AGX GPU driver](Apple-AGX-GPU-driver.md)
   - [Nova GPU Driver](Nova-GPU-Driver.md)
   - [NVMe Driver](NVMe-driver.md)

src/Android-`ashmem`.md

@@ -0,0 +1,5 @@
+# Android `ashmem` ("Anonymous Shared Memory Subsystem")
+
+The [`ashmem`](https://android.googlesource.com/kernel/common/+/refs/heads/android16-6.12/drivers/staging/android/ashmem_rust.rs) subsystem is a new shared memory allocator, similar to POSIX SHM but with different behavior and sporting a simpler file-based API.
+
+It is, in theory, a good memory allocator for low-memory devices, because it can discard shared memory units when under memory pressure.