Add kernel memory allocations to bugreport

Add the contents of /proc/allocinfo to the bugreport.
This file shows the current snapshot of all the memory allocated and the
number of allocations associated with kernel code locations.
The collection and processing of /proc/allocinfo is done by calling the
alloctop tool, to return the top entries that allocated most memory.
The kernel memory allocations entry is added to the bugreport only if
the /proc/allocinfo file exists.

As shown below, a rough measurement of the execution time of this
command on Cuttlefish shows an average of ~43 ms.
On the same device, the total number of characters generated by the
command is 6704, so ~6.6 KiB uncompressed.

```
vsoc_x86_64:/ # time for i in $(seq 1 1000); do
  alloctop --once --sort s --min 1 --lines 100 > /dev/null
done
    0m42.25s real     0m00.17s user     0m03.63s system

vsoc_x86_64:/ # alloctop --once --sort s --min 1 --lines 100 | wc -c
6704
```

Bug: 384777823
Test: adb shell dumpstate
Change-Id: Ib8f7eeebe5ad88fc608e2d84582ad32e1b069985
Signed-off-by: Alessio Balsini <balsini@google.com>