Update RenderEngineTest's buffer dumping file paths

Writing to `/data/file.ppm` was failing for me, even with `adb shell
setenforce 0`. Changing this to `/data/local/tmp/file.ppm` let the write
succeed, event without `adb shell setenforce 0`.

I chose `/data/local/tmp/` based on context clues from other tests, but
am happy with any directory, as long as we have write permission. After
feedback I've also changed the code to create the target directory if it
doesn't exist, which allows us to use `/data/local/tmp/RenderEngineTest`
and remove the `texture_out_` file name prefix.

Note: both `/data/` and `/data/local/tmp/` have the same permissions of
771, but they're owned by different users, `system` and `shell`
respectively.

I also tweaked the file name since tests parameterized by RenderEngine
type were appending e.g. `/0`.

Bug: N/A
Test: manual
Flag: EXEMPT test only
Change-Id: I3017b870ecb4d61cd90206128774d45b003e3cf0