Switch protoc javastream plugin to using explicit path

The previous PATH=$PATH:out/... logic was incorrect, since the version
in $PATH would override the one out. And now that we're limiting what's
accessible via $PATH, that caused an error.

Instead, ensure that all protoc plugins are specified explicitly with
--plugin, then remove the PATH modification.

Test: cd frameworks/base/cmds/am; mma    (with protoc-gen-javastream in $PATH)
Change-Id: I6690727504f67f84fdc95ed93eabdf3351e5cb0b