Create a pool of 20 measurement files per library for coverage.

Reduces disk space usage for commonly used libraries by merging coverage
data across processes.

Bug: 171338125
Test: Forrest run of coverage tests
Change-Id: I2b9e94871fc5d66971c5b7e725b296bcd721ccf2

Author: Oliver Nguyen

rootdir/Android.mk

@@ -69,7 +69,7 @@ endif
 
 EXPORT_GLOBAL_CLANG_COVERAGE_OPTIONS :=
 ifeq ($(CLANG_COVERAGE),true)
-  EXPORT_GLOBAL_CLANG_COVERAGE_OPTIONS := export LLVM_PROFILE_FILE /data/misc/trace/clang-%p-%m.profraw
+  EXPORT_GLOBAL_CLANG_COVERAGE_OPTIONS := export LLVM_PROFILE_FILE /data/misc/trace/clang-%20m.profraw
 endif
 
 # Put it here instead of in init.rc module definition,