Start thread pool in skia_dm on Android.

Android media framework is expected to be run with the background
thread pool for handling binder RPC calls.

Test: skia_dm does not hang with heif files.
Bug: 65463215 Bug: 72869975
Change-Id: I2333d5a13d6145e9468464fa93807851e3a17bc8
Reviewed-on: https://skia-review.googlesource.com/108141
Reviewed-by: Chong Zhang <chz@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
2 files changed