Stop creating unnecessary Context objects

Previously, the Context object created in the main() method was
initialized (ctx.Register()) for the main soong build. Build modes
that did not want that build modes that did not want that (symlink,
bp2build, apiBp2build) had to create their own context, or in the
case of runSymlinkForestCreation its own EventHandler. That was
very confusing.

This change avoids that by simply pushing the call to ctx.Register()
into the build modes that require it and that allows the other build
modes to use the main context and its EventHandler.

One point to note is that to ensure the exact same initialization as
before the runApiBp2build has to explicitly call the following:
    ctx.SetAllowMissingDependencies(false)
That is because the main context could have that set to true.

Bug: 257650737
Test: m nothing
Change-Id: Iee239fb87edf443fed65156fa14b8a30c89a2328