Installer: page refresh should refresh list of supported DBs

There's no need to cache _CompiledDBs's value from the first installer load.
Instead, the list should be regenerated on every page load, as it's merely being
used for displaying the user list of DBs to choose from.

This solves a common setup confusion, where user starts with no DB support, gets
a friendly error message, installs php5-mysql as requested, but refreshing the
page won't make the error go away.

Bug: 31533
Change-Id: I2e7e5b4f72e7c11706d491a205e202008e2511da
Merge "Installer: page refresh should refresh list of supported DBs"