Refactor around apexKeysText singleton

apexkeys.txt should list the installed apexes. But for now it lists all
apexes in the source tree. To fix this, each apex(or prebuilt/apexset)
will generate its own apexkey info and they'll be merged into a single
text only for installed apexes.

This change is a preparation before the upcoming change. We'll reuse the
apexKeyEntry stuff from the current implementation.

Bug: 304914238
Test: m blueprint_tests
Change-Id: If9d5d992e5e2b0120e017265d2590b1c55152f52