adbd: increase oom_score_adj for `adb shell`

Previously, processes started via `adb shell` have an oom_score_adj of
-1000, making them invisible to the oom killer. This makes running a
process that consumes all memory (e.g. by leaking in a loop) lead to
the entire rest of the system (including adbd, because of bad
heuristics in the kernel) being oom killed before getting to it.

Bug: http://b/63143027
Test: `adb shell cat /proc/self/oom_score_adj` with adb root
Change-Id: I59111134e36dc271adf4c1dd4bd4400d4fe6aee0
(cherry picked from commit 2de2ade3)