OMS: rename methods to include package downgrades

The overlay manager service handles package upgrades and downgrades the
same way: all it cares about is a difference in versions, not if the new
version has a higher or lower value. In both cases, the same family of
methods are called: onPackageUpgrading and onPackageUpgraded.

Rename onPackageUpgrading and onPackageUpgraded to onPackageReplacing
and onPackageReplaced to better highlight that both upgrades and
downgrades are supported.

Test: atest OverlayDeviceTests OverlayHostTests
Change-Id: I04c5c550f504114c22bb30d35c9f2809194d2543