screencap: always exit with _exit().

Delay starting the threadpool until necessary,
and once started, always exit with _exit().

Bug: 36066697
Bug: 62956201
Test: mma, screencap -p
Merged-In: I0b9ab542f6434f57b498cd7dc41219ef57e36721
Change-Id: I0b9ab542f6434f57b498cd7dc41219ef57e36721

(cherry picked from commit 4d125c1df6b7e4dbc3a788ecce5434970e676b06)

Change-Id: Ie79ead94c62df0c4057d7fc6735abe9dcd1e940f
1 file changed