When installing java, add it to PATH.

Its possible we were not adding this to directory to `PATH` to avoid
message with the user's already installed java.  However this seems
inconsistent given:

1. We do set `JAVA_HOME`, which is already interfering with any java
   installation the user might have.
2. We do add node to that PATH, overriding any node installation the
   user might have.

This change makes removal of `JAVA` config setting from emscripten
feasible since it doesn't have to handle this strange half-configured
java installation: https://github.com/emscripten-core/emscripten/pull/15055

As a followup we could consider not install java at all anymore since it
doesn't have any usage in modern emscripten (at least not on windows
where the native version of closure-compiler should always be
available).

emsdk_manifest.json

diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index e6c5dc0..83c8fda 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -359,6 +359,7 @@
     "arch": "x86",
     "windows_url": "portable_jre_8_update_152_32bit.zip",
     "activated_env": "JAVA_HOME=%installation_dir%",
+    "activated_path": "%installation_dir%/bin",
     "activated_cfg": "JAVA='%installation_dir%/bin/java%.exe%'"
   },
   {
@@ -368,6 +369,7 @@
     "arch": "x86_64",
     "windows_url": "portable_jre_8_update_152_64bit.zip",
     "activated_env": "JAVA_HOME=%installation_dir%",
+    "activated_path": "%installation_dir%/bin",
     "activated_cfg": "JAVA='%installation_dir%/bin/java%.exe%'"
   },
   {