adb: fdevent: add fdevent_context_epoll.

Implement an epoll-backed fdevent_context, that reduces overhead when
we're polling many file descriptors. FdeventTest.smoke goes from ~5.2s
to ~3.3s when run on the host (after this patch's modification to change
it from chaining 10 file descriptors together to 512).

Test: adb_test on host
Test: adbd_test on blueline
Test: test_adb.py
Test: test_device.py
Change-Id: Iacf0093aa7bebea31e447c2cb012af72d8c3297e
diff --git a/adb/Android.bp b/adb/Android.bp
index 06cfcbf..57872b0 100644
--- a/adb/Android.bp
+++ b/adb/Android.bp
@@ -142,6 +142,10 @@
     "sysdeps/posix/network.cpp",
 ]
 
+libadb_linux_srcs = [
+    "fdevent/fdevent_epoll.cpp",
+]
+
 libadb_test_srcs = [
     "adb_io_test.cpp",
     "adb_listeners_test.cpp",
@@ -170,12 +174,11 @@
 
     target: {
         linux: {
-            srcs: ["client/usb_linux.cpp"],
+            srcs: ["client/usb_linux.cpp"] + libadb_linux_srcs,
         },
         darwin: {
             srcs: ["client/usb_osx.cpp"],
         },
-
         not_windows: {
             srcs: libadb_posix_srcs,
         },
@@ -342,7 +345,7 @@
     // libminadbd wants both, as it's used to build native tests.
     compile_multilib: "both",
 
-    srcs: libadb_srcs + libadb_posix_srcs + [
+    srcs: libadb_srcs + libadb_linux_srcs + libadb_posix_srcs + [
         "daemon/auth.cpp",
         "daemon/jdwp_service.cpp",
     ],