Detect leaks on host by default.

Right now, leaks are disabled globally on host. This is problematic
because:
a). people write leaks in new code
b). if we fix leaks in old code, it doesn't get tested
c). fixing leaks in old code is harder because when we set
    ASAN_OPTIONS="" locally, other targets start hitting errors,
    and we have to set __asan_default_options there

Fixes: 141313466
Test: host ASAN build passes
Change-Id: I89298c7518c016e6a30884c75d7b791db16a5217

envsetup.sh

@@ -33,9 +33,7 @@ Invoke ". build/envsetup.sh" from your shell to add the following functions to y
 - refreshmod: Refresh list of modules for allmod/gomod.
 
 Environment options:
-- SANITIZE_HOST: Set to 'true' to use ASAN for all host modules. Note that
-                 ASAN_OPTIONS=detect_leaks=0 will be set by default until the
-                 build is leak-check clean.
+- SANITIZE_HOST: Set to 'true' to use ASAN for all host modules.
 - ANDROID_QUIET_BUILD: set to 'true' to display only the essential messages.
 
 Look at the source to view more functions. The complete list is:
@@ -333,7 +331,6 @@ function set_stuff_for_environment()
     export ANDROID_BUILD_TOP=$(gettop)
     # With this environment variable new GCC can apply colors to warnings/errors
     export GCC_COLORS='error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01'
-    export ASAN_OPTIONS=detect_leaks=0
 }
 
 function set_sequence_number()