Decoder: add support for KEEP_THREADS_ACTIVE

Current design of HEVC Decoder creates new threads
to process each frame. This CL adds support to reuse
the threads across frames.

Test: manual
Bug: 167126301

Change-Id: Ic57133b25203dae9ed5d3d974e6137a733531da7
diff --git a/Android.bp b/Android.bp
index c177cfd..7ad4216 100644
--- a/Android.bp
+++ b/Android.bp
@@ -15,6 +15,8 @@
         // common/x86/ihevc_sao_ssse3_intr.c: implicit conversion from
         // 'int' to 'char' changes value from 128 to -128
         "-Wno-error=constant-conversion",
+        // #KEEP_THREAD_ACTIVE is experimental
+        "-UKEEP_THREADS_ACTIVE",
     ],
 
     export_include_dirs: [