Merge latest API into single file before checking compatibility

Previously, the `frameworks-base-api*-current-compat` modules would
perform a compatibility check between the latest released API
defined within multiple signature files and the current API
defined within a single signature file. That breaks on the latest
version of Metalava which has some more checks to ensure that the
API surfaces being compared are compatible.

Currently, the checks are heuristic based and essentially compare the
number of signature files for each API to catch comparing say public
API with one file against system API defined in two files (public +
system delta). This change works around the problem by merging the
files for the latest previously released API into a single file just as
is already done for the current API.

Longer term the signature files will contain information about which
API they belong to which will allow Metalava to make a much more
meaningful check at which point the affected modules can be simplified.

Bug: 374096206
Test: m checkapi
Change-Id: I934e7ac664b6b7fc9150a14b0787c0e706ccc3cd