Vagrantfile: make sure ANDROID_HOME and PATH make it as variable names to bashrc

When we write the new path to include the android SDK, the variables are
expanded when the shell is running, resulting in the wrong export for
PATH being created in bashrc because for the running shell ANDROID_HOME
is undefined:

export PATH=/platform-tools:/tools:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Escape the $ so that the variables are expanded when bashrc is run.
With this change, we end up with this in bashrc:

export PATH=$ANDROID_HOME/platform-tools:$ANDROID_HOME/tools:$PATH